amine llto cond

7/31/2019 Amine LLTO Cond

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.Solid State Ionics 144 2001 5157

www.elsevier.comrlocaterssi

Ionic conductivity, lithium insertion and extraction of lanthanumlithium titanate

C.H. Chen, K. Amine)

Chemical Technology Diision, Electrochemical Technology Program, Argonne National Laboratory, 9700 South Cass Aenue,

Bldg. 203-C110, Argonne, IL 60439, USA

Received 6 November 2000; received in revised form 8 May 2001; accepted 18 May 2001

Abstract

.The lithium ionic conductivity and electrochemical stability of perovskite La Li TiO LLTO have been2r3yx 3x 3

determined with AC impedance spectroscopy, cyclic voltammetry and galvanostatic cycling. Ionic conductivity of

La Li TiO and La Li TiO pellets sintered from four different powders was measured in the temperature range0.55 0.35 3 0.57 0.29 3

from 30 to 110 8C. Bulk conductivity was found to be closely related to the calcination temperature of the powders. Pellets

from 1100 8C-calcined powders had higher bulk conductivity than from 1200 8C-calcined powders. The grain-boundary

conductivity was mainly determined by the sample composition. The activation energies were 0.140.18 eV for bulk

conduction and 0.410.43 eV for grain-boundary conduction. Lithium was intercalated into LLTO below about 1.8 V vs. Li.

With addition of acetylene black, about 0.48 Li was reversibly inserted into and extracted out of La Li TiO . A phase0.55 0.35 3

transition is proposed to take place during the lithium insertion. Published by Elsevier Science B.V.

Keywords: Ionic conductivity; Lithium insertion; Lanthanum lithium titanate

1. Introduction

The development of all-solid-state lithium-ion bat-

teries has received considerable attention because of

their possible application to the new generation of

energy sources in microelectronic and informationw xindustry 14 . Some obvious advantages over the

current lithium-ion batteries can be expected with the

liquid-free batteries. These include thermal stability,

absence of leaks and pollution, resistance to shocks

and vibrations, and a possible large electrochemicalwindow allowing the use of 5-V cathodes. However,

the main impediment is finding a sound solid elec-

)

Corresponding author. Tel.: q1-630-2523838; fax: q1-630-

2524176. .E-mail address: [email protected]. K. Amine .

trolyte that has a reasonably high lithium ionic con-

ductivity and good stability. To date, the fastest

lithium-ion-conducting electrolytes are the per- .ovskite-type ABO lanthanum lithium titanates3

. w xLa Li TiO LLTO and their variants 5 9 .2r3yx 3x 3In the structure of these materials, there are a sub-

stantial number of A-site vacancies through which

lithium can transport. At room temperature, they

possess a bulk conductivity of 10y3 Srcm and a

grain-boundary conductivity of 10

y4

10

y5

Srcm.These conductivities are comparable with those of

the commonly used liquid electrolyte. However, therew xare reports that La Li TiO 10 and La Li0.56 0.33 3 0.57 0.29

w xTiO 11 can intercalate lithium in the structure and3introduce electronic conductivity at a potential below

about 1.7 V vs. Li. The maximum lithium uptake

was found to be equal to, or smaller than, the

0167-2738r01r$ - see front matter. Published by Elsevier Science B.V. .P I I : S 0 1 6 7 - 2 7 3 8 0 1 0 0 8 8 4 - 0


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( )C.H. Chen, K. Amine rSolid State Ionics 144 2001 515752

number of A-sites available in the perovskite struc-

ture. This means that the use of these materials as

electrolyte is probably unsuitable while metallic

lithium or lithiated carbon is employed simultane-

ously as an anode. As an alternative, a less energetic

anode such as the 1.5-V electrode Li Ti O may be4 5 1 2w xused with some sacrifice of battery energy 12,13 . In

our study, the ionic conductivity of four LLTO

samples with either different compositions or differ-

ent synthesis conditions was compared in order to

find an optimum for different application. Further-

more, the electrochemical stability of LLTO was

checked with button-cell technology, and the maxi-

mum lithium uptake was found to be more than the

number of available A-site vacancies in the per-

ovskite.

2. Experimental

A solid-state reaction procedure was adopted to

prepare the perovskite lanthanum lithium titanate

with two target compositions: La Li TiO and0.55 0.35 3La Li TiO . The compounds La O , Li CO ,0.57 0.29 3 2 3 2 3and TiO were mixed together with ethanol and2ball-milled for 15 h. The mixtures were calcined at

either 1100 or 1200 8C, followed by drying of

ethanol. After grinding, four powders were obtained:

La Li TiO calcined at 1100 8C, La Li0.55 0.35 3 0.55 0.35

TiO calcined at 1200 8C for 12 h, La Li TiO3 0.57 0.29 3calcined at 1100 8C, and La Li TiO calcined at0.57 0.29 31200 8C. X-ray diffraction analysis confirmed that

these powders were pure perovskite phase. The pow- .ders were pressed into pellets 15 mm in diameter

and sintered at 1200 8C for 12 h in air. The sintering

temperature of 1200 8C was selected because a ther-

mal analysis of these powders indicated that they

melted at temperatures above 1250 8C, although

sintering at 1350 8C has been reported in the litera-w xture 5 . The density of the pellets was measured by

the Archimedes method.

Two sides of the pellets were sputtered with a thin

gold layer for conductivity measurement. A CHI660model Electrochemical Workstation CH Instru-

.ments was used to acquire the AC impedance spec-

tra of these LLTO samples in the frequency range

from 1 mHz to 100 kHz, and the temperature range

from room temperature to 110 8C.

The La Li TiO powder calcined at 1100 8C0.55 0.35 3was made into electrode laminates to check the

electrochemical stability. Two laminates with and

without carbon addition were prepared on 15-mm-

thick copper foil by the tape-casting technique. The

carbon-free laminate was composed of 92 wt.% .LLTO and 8 wt.% polyvinylidene PVDF , while the

carbon-containing laminate was composed of 84

wt.% LLTO, 8 wt.% acetylene black, and 8 wt.% .PVDF. Button cells size 2032 were made using the

punched laminates as cathode, lithium foil as anode, .and 1 M LiPF in ethylene carbonate EC rdiethyl6

.carbonate DEC as electrolyte. The cycling of these

cells was performed on a Maccor cycler. Cyclic

voltammetry and AC impedance spectroscopy were

also used to characterize the cells on the CHI660

model Electrochemical Workstation. For compari-

son, a button cell using a laminate composed of 80

wt.% acetylene black and 20 wt.% PVDF as elec-trode was also tested.

3. Results and discussion

3.1. Ionic conductiity and actiation energy of LLTO

As a representative of the AC impedance spectra

of the sintered pellets, Fig. 1 shows the results for

La Li TiO calcined at 1100 8C and sintered at0.55 0.35 3

1200 8C. Each spectrum consists of a semicircle inthe high-frequency range and a straight line in the

.low-frequency range Fig. 1a . In addition, there is

an intercept at the high-frequency end of the semicir- .cle Fig. 1b . This impedance spectrum is typical for

w xa pure ionic conductor with blocking electrodes 14

and consistent with that observed by Fragnaud andw xSchleich 1 . In fact, the electronic conductivity of

LLTO estimated from a DC measurement is in the

order of 10y7 Srcm at room temperature. In princi-

ple, a semicircle between the origin and the intercept

would be observed if much higher frequencies are

used in the experiment. The high-frequency limit

should be as high as 0.11 GHz because the time .constant of this bulk process RC is 1 ns withgeo

.assuming a geometric capacitance C of 101 pF.geoThe high-frequency limits used in this study 100

. .kHz and in Inaguma et al.s 13 MHz are obviously

not sufficient. The intercept represents the total ionic


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( )C.H. Chen, K. Amine rSolid State Ionics 144 2001 5157 53

Fig. 1. AC impedance spectra of a La Li TiO pellet at0.55 0.35 3different temperatures after it had been calcined at 1100 8C and

. .sintered at 1200 8C: a full spectra and b high-frequency part.

resistance of LLTO grains, while the semicircle is

usually assigned to the relaxation process in LLTO

grain boundaries. The straight line is related to the

lithium-ion diffusion occurring at the interface be-

tween LLTO and the sputtered gold layers. With

increasing temperature and thus causing a faster

diffusion, the interface tends to shift from an infinite

space to a finite space. Therefore, more charge accu-

mulation process, which corresponds to a capacitive

behavior, is involved at higher temperatures. Hence,

the angle of the straight line in Fig. 1a increases with

temperature. Under AC measurement conditions, no

detectable lithium deposition on gold due to this

diffusion process is observed. These impedance spec-

tra can be fit with such a simple equivalent circuit 2 y3 y4 .satisfactorily x around 10 10 . The obtained

grain-boundary capacitance is in the order of 10 nF,

which is typical for polycrystalline ionic conductors.

Fig. 2 presents Arrhenius plots of the bulk con-

ductivity and grain-boundary conductivity of these

pellets at different temperatures. The room tempera- .ture 30 8C conductivity and activation energy are

also given in Table 1. It can be seen that, consistentw xwith the literature data 5,6 , the bulk conductivity of

LLTO at 30 8C is around 10y3 Srcm and the

grain-boundary conductivity at 30 8C is above 10y5

Srcm. By comparing the results from the four dif-

ferent pellets, one can see that the bulk conductivi-

.Fig. 2. Arrhenius plots of the lithium ionic conductivity: a bulk .conductivity and b grain-boundary conductivity. Four pellets:

. A La Li TiO calcined at 1100 8C diamonds with solid0.55 0.35 3. . line , B La Li TiO calcined at 1200 8C triangles with0.55 0.35 3

. . solid line , C La Li TiO calcined at 1100 8C squares with0.57 0.29 3. . dashed line , and D La Li TiO calcined at 1200 8C crosses0.57 0.29 3

.with dashed line .


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Table 1

Density, ionic conductivity and activation energy of La Li TiO and La Li TiO0.55 0.35 3 0.57 0.29 3

Samples Composition T d s E s Ecalcination bulk,30 a,bulk gb,30 a,gby3 y3 y5 . . . . . .8C g cm 10 Srcm eV 10 Srcm eV

A La Li TiO 1100 4.50 1.19 0.14 2.64 0.430.55 0.35 3B La Li TiO 1200 4.37 0.89 0.16 2.71 0.410.55 0.35 3C La Li TiO 1100 4.80 1.17 0.15 4.68 0.430.57 0.29 3

D La Li TiO 1200 4.48 0.66 0.18 3.71 0.420.57 0.29 3

.ties of two pellets A and C from the 1100 8C-

calcined powders are very close, yet higher than .those of the other two pellets B and D from 1200

.8C-calcined powders Fig. 2a . It appears that the

pellet density has an important effect on the bulk

conductivity. This condition is usually true for

grain-boundary conductivity because a high density

suggests the presence of relatively thin grain-

boundaries. However, the real reason might be re-lated to the composition change of the samples. It is

known that at temperatures above 900 8C lithium .oxide Li O can be evaporated from lithium-con-2

w xtaining solid solutions such as lithium zirconates 15w xand Li Ni O 16 . More lithium loss could bex 1yx

expected to take place in the samples B and D than

A and C, leading to the difference in bulk conductiv-

ity. The activation energy for the bulk conduction is

0.14 0.18 eV, which is considerably smaller thanw xInaguma et al.s 5 result of 0.40 eV for this low

temperature range. However, Inaguma et al. alsoobtained another activation energy, 0.15 eV, for the

bulk conduction in the temperature range from 100

to 400 8C. The activation energy for the grain-

boundary conduction is 0.410.43 eV, which agrees

well with Inaguma et al.s results.The results of grain-boundary conductivity Fig.

.2b show that the lithium ions diffuse faster in the

grain boundaries of La Li TiO pellets than in0.57 0.29 3those of La Li TiO pellets. This may be at-0.55 0.35 3tributed to the compositional difference of the grain

boundaries because different bulk compositions may

lead to different segregation kinetics and, therefore, a

different composition at the grain boundaries. For the

La Li TiO , the pellet from 1100 8C-calcined0.57 0.29 3powder has higher conductivity than that from 1200

8C-calcined powder because the density of former

sample is greater. Nevertheless, very little difference

is observed for the composition La Li TiO .0.55 0.35 3

3.2. Lithium insertion and extraction in LLTO

Fig. 3 shows the cyclic voltammogram of a freshly

made 2032 button cell using the laminate consisting

of 92 wt.% La Li TiO and 8 wt.% PVDF as0.55 0.35 3cathode. During the initial lithiation half-cycle,

lithium can be intercalated into LLTO below the

potential 1.8 V. This finding is in agreement with the

literature reports, although there is a small discrep-w xancy about the intercalation onset potential 10,11 .

However, no extraction peak is observed during the

first charge shown in Fig. 3. Furthermore, no signifi-

cant active signals appear in the subsequent cycles.

This means that the lithium insertion process seems

irreversible when no conducting additive is used in

the sample. This irreversibility is likely caused by

the very high over-potential due to the very poor

electronic conductivity of the sample. Similar phe-

nomena were observed in the lithiation of w xhollandite-type TiO 17 . Nevertheless, this process

2is reversible after adding acetylene black in thelaminate to enhance the electronic conductivity see

.below .

Fig. 3. Cyclic voltammogram of a cell using a LLTO laminate

consisting of 92 wt.% La Li TiO and 8 wt.% PVDF vs. Li.0.55 0.35 3The scan rate was 1 mVrs. Cycle numbers are indicated.


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.Fig. 4. Cyclic voltammograms of a cell using a LLTO laminate

consisting of 84 wt.% La Li TiO , 8 wt.% acetylene black,0.55 0.35 3 .and 8 wt.% PVDF vs. Li and b cell using a laminate consisting

of 80 wt.% acetylene black and 20 wt.% PVDF vs. Li. The scan

rate was 1 mVrs. The first cycle is indicated.

Fig. 4a shows the cyclic voltammograms of the

cell using a cathode consisting of 84 wt.% La0.55Li TiO , 8 wt.% acetylene black, and 8 wt.%0.35 3PVDF and Li anode. The result is quite different

from the voltammogram for carbon-free laminate .Fig. 3 . In the first lithiation half-cycle, two signifi-

cant intercalation steps, from 1.8 to 1.1 V and from

0.6 to 0 V, and a small peak around 0.9 V are

observed. On the first delithiation half-cycle and the

subsequent cycles, four well-resolved peaks appear

quite reversibly in each chargedischarge cycle. The

peak potentials are approximately 1.5, 1.04, 0.55 and

0 V for lithium insertion during the anodic scans,

and, correspondingly, approximately 1.67, 1.2, 0.77,

and 0.55 V for lithium extraction during the cathodic

scans. Therefore, the averages of the potentials for

insertion and corresponding extraction peaks are ap-

proximately 1.6, 1.1, 0.66, and 0.27 V, respectively.

On the other hand, the cyclic voltammogram of a

cell using an acetylene black laminate as cathode .Fig. 4b has peaks at 1.1 and 0.27 V, which are

regarded as the contribution from insertion and ex-

traction of lithium in and out of acetylene black.

Therefore, the 1.6 and 0.66 V steps in Fig. 4a can be

ascribed to the lithium insertionrextraction in LLTO.

In addition to the reversible lithium insertion and

extraction steps shown in Fig. 4a, the charge quanti-

ties involved in the two lithiation steps, from 1.8 to

1.1 V and from 0.6 to 0 V, on the first anodic scan

are obviously substantially more than those in the

subsequent cycles. Similar to usual carbon anodes in

electrochemical lithium cells, the step from 0.6 to 0

V probably includes the formation of thin solid .electrolyte interface SEI layers on the acetylene

black particles due to decomposition of electrolyte at

low potential. A similar big step is also observed .without mixing LLTO in the laminate Fig. 4b . The

possibility of forming SEI layers on LLTO particles

is very small because of their poor electronic con-

ductivity. The step from 1.8 to 1.1 V is apparently

only related to the lithium insertion in LLTO. More

charge quantity involved in this suggests that only

part of the lithium intercalated in the first lithiation

half-cycle can be reversibly cycled afterwards.

Figs. 5 and 6 show the galvanostatic cycling

results from cells without and with acetylene-black-

containing laminates, respectively. In general, theyare very much consistent with the cyclic voltamme-

try results discussed before. As shown in Fig. 5,

Fig. 5. Cycling curve of a cell using a LLTO laminate consisting

of 92 wt.% La Li TiO and 8 wt.% PVDF vs. Li. Current0.55 0.35 3density is 0.0625 mArcm2. The spikes on the curve resulted from

30-s interruption.


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Fig. 6. Cycling curve of a cell using an LLTO laminate consisting

of 84 wt.% La Li TiO , 8 wt.% acetylene black, and 8 wt.%0.55 0.35 32 .PVDF vs. Li. Current density is 0.0625 mArcm : a first 19

.cycles and b first 4 cycles.

lithium may be inserted in the LLTO electrode in the

initial lithiation, but is difficult to extract from the

acetylene-black-free cell. As shown in Fig. 6, lithium

can be inserted as well as extracted in the acetylene-

black-containing cell. Nevertheless, the initial lithia-

tion capacity obtained is considerably higher than the

subsequent cycle capacity.

As discussed before, the cell capacity in the acety-

lene-black-containing cell is partially contributed by

acetylene black. From the half-cells using a cathode

consisting of 80 wt.% acetylene black and 20 wt.%

PVDF, we obtained an initial intercalation capacity

ranging from 445 to 676 mA hrg, and a reversible

capacity 160 mA hrg in subsequent cycles for acety-

lene black in the voltage window between 2.5 and 0

V. After subtracting the contribution from acetylene

black, we determined the cell capacity and its

converted equivalent number of insertedrextracted

lithium per formula of La Li TiO . Fig. 7 shows0.55 0.35 3these results, together with the cycling results of the

acetylene-black-free cell. It can be seen that about

0.48 Li may be inserted into one formula of

La Li TiO in the initial lithiation half-cycle in0.55 0.35 3the acetylene-black-free cell. For the acetylene-

black-containing cell, about 0.750.9 Li can be ini-

tially inserted into LLTO; however, only 0.48 Li can

be reversibly inserted into and extracted out of one

formula of La Li TiO in subsequent cycles.0.55 0.35 3 .These values 0.75 0.9 and 0.48 are well above the

.number of available vacant perovskite A-sites 0.1w xin La Li TiO . Other researchers 6,7 did not0.55 0.35 3

observe this effect, partly because they did not ex-

tend the voltage window to 0 V. We propose a

possible phase transition mechanism to explain this

phenomenon. In the initial lithiation half-cycle, the .perovskite structure ABO might be transformed to3

A BO monoclinic phase after filling all the vacant2 3perovskite A-sites. This A BO phase could be sim-2 3

w xilar to rechargeable Li RuO 18,19 . A point of2yx 3inflection at potential of about 0.68 V is noticed on

the lithiation curve of the LLTO laminate in Fig. 5

and indicated by an arrow. This point of inflection

could be caused by the proposed phase transition at

the intercalation of about 0.24 Li. Further composi-

tional and structural analyses are needed to confirm

the occurrence of this phase transformation and to

explain why it occurs at the intercalation of 0.24 Li

Fig. 7. Capacity and corresponding number of Li ions inserted .into or extracted out of LLTO: charge triangles and discharge

. .crosses of the acetylene-black-free cell, and charge squares and .discharge diamonds of the acetylene-black-containing cell.


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not 0.1 Li as predicted by La Li TiO compo-0.55 0.35 3.sition .

4. Conclusions

Pellets of La Li TiO and La Li TiO0.55 0.35 3 0.57 0.29 3sintered from four different powders were studied

with AC impedance spectroscopy. Bulk conductivity

was found to be closely related to the calcination

temperature of the powders. Pellets from 1100 8C-

calcined powders had higher bulk conductivity than

those from 1200 8C-calcined powders. The grain-

boundary conductivity was mainly determined by the

sample composition. The activation energies were

0.140.18 eV for bulk conduction and 0.410.43 eV

for grain-boundary conduction.

Lithium was intercalated into LLTO below about

1.8 V vs. Li. With addition of acetylene black, about0.48 Li was reversibly inserted into and extracted out

of La Li TiO . A phase transition is proposed0.55 0.35 3to take place during the first lithium insertion step.

Because of the lithium insertion into LLTO at the

potential below 1.8 V vs. Li, and thus the introduc-

tion of electronic conductivity, LLTO is obviously

not a suitable electrolyte material in lithium ion

batteries that use either metallic lithium or lithiated

carbon as anode material. It can probably be used as

electrolyte materials when some high potential oxide

materials, for instance, Li Ti O are used as anode.4 5 12

Acknowledgements

This study was supported by the Laboratory Di- .rector R & D LDRD program of Argonne National

Laboratory.

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