New fast solid lithium ion conductors at low and imtermediate temperatures

Solid State Ionics 18 & 19 (1986) 529-533 North-Holland, Amsterdam

529

NEW FAST SOLID LITHIUM ION CONDUCTORS AT LOW AND IMTERMEDIATE

B. SCHOCH,

E. HARTMANN,

Max-Planck-Instltut

TEMPERATURES

and W. WEPPNER

fur Festk~rperforschung,

D-7000 Stuttgart

80, Fed. Rep. Germany

Three approaches for practically useful new solid lithium electrolytes are presented. Combination of two binary lithium salts provides materials which are stable against reaction with lithium and decompose at intermediate values of the decomposition voltages of the two binary salts. Results are given for systems based bn the ionic conductor Li2S and lithium halides. A small number of ternary lithium compounds exist which contain a second type of cations with a higher bind'ing energy to the anion than lithium. These materials are also stable with lithiums. Results obtained for the compounds LiMgN and Li3AIN 2 which crystallize both in an antifluori~te type structure are reported. The third class of solid lithium ion conductors are addition compounds formed from lithium halides and alcoholes. LiI.4CH30H shows a room temperature conductivity of 2.2xI0 -4 ~-Icm-~ and an activation enthalpy of 0.51 eV.

binding energy and forms the more stable binary

I. INTRODUCTION Fast

solid

attracted much of

the

lithium

conductors

have

lithium compound 4. The search for fast thermodynamically stable

interest in recent years in view

development

batteries.

ion

of

high

energy

density

The low atomic weight of lithium and

solid

lithium

strategies

ion

than

conductors

in

the

case

requires of

the

other

classical

the high negative Gibbs energies of formation of

silver or copper ion conductors. Recent investi-

many

in

gations

in

ches. The first is based on lithium double salts

the

lithium course

this

compounds

of

respect

which may

discharge

are

. In spite of

be

very

formed

promising

the fact

that many

and

have

has

followed

already lithium

three

indicated

several

practically

useful

are suitable for certain types of appl~cations,

which crystallizes in a lithium deficient antifluorite

cally

voltage

stable

solid

lithium

electrolytes

with

type

conductors,

approa-

solid lithium ion conductors became known which

there is still a general need for thermodynami-

ion

different

structure

and

e.g.,

LigN2C13

decomposes

at

a

higher than 2.52 V at 100 °C5. The ter-

high ionic conductivity. Previous investigations

nary

have

lithium salts that are both stable with lithium

often

shown

either

high

conductivity

and

compounds

prepared

binary

ionic conductivity (LiI2'3). Using a second type

stable with lithium. The value of the decomposi-

of

tion voltage is by general thermodynamic considerations

in

compounds.

the

successful

approach

for

fast

silver

or

The

copper ion conductors, reduces in most cases the stability is

preferably

effect dynamic the

against

is

due

used to

the

stability of

salt of

replace

reactions

the

as

with

anode

lithium salts

species

which

for

the

binary

investigation of this class of materials

is extended

to

the

systems Li2S

- lithium ha-

This

of the antifluorite type structure which appears

compared to

the

values

also

thermo-

Lithium tends to with

the

always)

lides. Li2S was taken into consideration because

material.

generally higher

the other cation. cationic

lithium

between

(practically

two

and

disorder or to form a new ternary compound, like

therefore

from

low s t a b i l i t y (Li3N I) or high stability and low

(aliovalent) cation in order to increase the

are

are

smaller

0 167-2738/86/$ 03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

to was

be

favorable

chosen

stability

for

because (6.1

fast of

ionic

the

high

transport.

LiF

thermodynamic

V decomposition voltage at room

530

B. Schoch et al. / N e w fast solid lithium ion conductors

temperature). The phase equilibria of the inves-

predominantly

tigated systems are not

dent from Hebb-Wagner polarization measurements 7

known from

the litera-

ionic which

is most

clearly evi-

ture and are determined by differential thermal

using reversible

analysis.

conductivity as a function

and ionically blocking inert molybdenum electro-

of temperature and composition provided a conve-

des. The magnitude of ionic conductivity depends

Also,

the

"LiAI",AI reference electrodes

nient analytical tool for phase diagram studies.

strongly by up to 2 orders of magnitude

In

kinetically

to 800°C) on the process of preparation and the

very slow processes may be readily detected. In

gas composition under which the measurements are

contrast

addition, the

to

dynamic

two-phase

present

techniques,

mixtures

investigation

information

on

the

were

for

included

further

electrical

in

performed.

general

conductivity

of

of

the

small

ductivity

number

of

ternary

is

generally

lower

agreement with earlier results of a higher conif

lithium

electrodes

rather

than

chemically inert molybdenum sheets are used 8 but

The second approach is based on the investigation

conductivity

at a higher sulfur partial pressure. This is in

e l e c t r o l y t e s 6.

composite

The

(at 300

lithium

may not be understood from the point of view of

compounds which are formed from a binary lithium

a lithium vacancy mechanism.

salt and one of the very few more stable other

may

binary

process along octahedral sites. The conductivity

salts

with

the

same

anion.

Results

are

of

reported for the systems Li3N-MgN and Li3N-AIN. The large

last

group

variety

of

of

materials

addition

consists

compounds

of

which

point

to

an

These observations

interstitial

polycrystalline

sintered

type

conduction

Li2S-

pellets

(99.9%) was found to be in the range from Ixi0 -s

a

to

are

1.5xi0 -3

Q-icm-1

ductile

The

and

amines

crystalline

have

or

other

materials

generally

low

organic

eV.

Li~S

has

a

very

with the same cubic fluorite structure. Mixtures

points

of LizS and Li20 show regular intermediate con-

are

melting

1.2

activation

higher conductivity than Li20 which crystallizes

alcoholes,

to

°C with

salts

and

0.9

400

formed between lithium halides or other lithium

compounds.

between

at

enthalpies

which may be an indication of mobile ionic spe-

ductivities

cies in the lattice at ambient temperatures.

enhancement.

without

2. TERNARY LITHIUM DOUBLE SALT ELECTROLYTES

salt type structure.

any

markedly

structured

All lithium halides crystallize with a rock-

Li2S

crystallizes

structure the

and

shows

thermodynamic

with an

an

stability

voltage of 3 V at 400 °C) . The

antifluorite

intermediate

value

corresponding at

25

°C

marized in Table I. The compounds were mixed in

type

increments

for to

2.27

V

2.18

type

of conduction is

of

10

m/o

(mol-%).

Additionally,

5

and 95 m/o were used. The mixtures were annealed

a

decomposition

Experimental data are sum-

under

(or

Cu

purified argon

catalysts,

p2Os,

gas

(oxidized and reduced

NaOH,

molecular sieve)

and

Table I. Compilation of the investigated lithium salt systems, the quality of the starting materials, the preparational parameters and eutectic melting temperatures. Also, the combinations of all halides were investigated.

Li-salt (a)

mesh size

purity I%]

Li2S

-200

99.9

Li-salt (b)

LiF LiCl LiBr LiI

mesh size

purity [%]

annealing temp. [°C]

-325 -60 -60 -8

99.9 99.8 99.8 99.9

600 550 500 400

annealing time [hrs]

8 8 8 8

-

10 10 10 10

eutect. melting temp.

[°C]

580 530 445

B. Schoch et al. / New fast solid lithium ion conductors

sintered

in covered molybdenum

perature

DTA

up

to

1400°C

performed

in addition to

X-ray

phase

examination

investigations.

for

5OO

under

argon gas using niobium ampoules Guinier

4192A

using

a

impedance

microprocessor

analyzer

controlled

and techniques

cribed earlier 9.

F-0

All systems melt eutectically at the tempera-

is

formed.

conductivities

Two

selected

as a

function of temperature are shown in Figs.

I and

2

for

the

as

the two-phase

systems

respectively. conductivity

A as

Li2S-LiBr

"regular" a

lithium

and

LizS-

variation

function

observed for Li2S-LiBr, is enhanced

(doped)

of

composition

mo[-% LiI

\ \\\\

[m,p.Li[~05~

~o?Og0

--eutekt. Temp.

qO -95

I

I

=

1.6

1.2

i

\

I

2

=

2.4

10"~-3[ K -I] T

2.8

.

the is

conducting binary

salt for 30 and 40 m/o LiI

ev

LiI,

for

whereas the conductivity

aver the superior

-2

the

mixtures

well

pure

of

com-

as

the

examples

binary

pounds

of

Li 2 S - LiI

;\\)

0

tures listed in Table I. No ifftermediate ternary phase

I

\

\

T

as des-

100

I

\

E

HP

200

I

X

T

The conductivity was measured by

T

MHz

I

i

diagram

ac impedance in the frequency range from 5 Hz to 13

T[°C] 300

boats. High tem-

was

531

FIGURE 2 Ionic conductivity of mixtures of LiaS and LiI. Higher values than for the pure compounds are observed at 30 and 40 mol-% LiI.

in the case of LiaS-LiI

(maximum at 40 m/o LiI). The enhan-

cement of the two-phase mixtures is explained by T[°C]

" oo

,oo

the conduction along space charge regions due to increased

cncentrations 6 similar to the 10 effect known for electronic conductors

Li2S_LiB r

defect

As a general

rule,

a balance

exists between

the ionic motion and the thermodynamic stability for

T

-2 /

\

k k k ~

simple

mol-%

tivity

LiBr

low

0

and similar

is high

and

vice

if the versa

inversely related

structures.

The

decomposition

since

both

conduc-

voltage

parameters

is are

to the binding of the ions in

the lattice.

N Lm..p.L!Br......

~'k~ °?o6°

temp. -4/ 1.2

=

I

1.4

=

I

1.6

3. TERNARY

1020 =

10.__.. 3 [K-l] T

I

1.8

STABLE

LITHIUM

ION

CONDUCTORS

WITH

TWO TYPES OF CATIONIC SPECIES =

I

The Gibbs

2.0

.

and

AIN

are

respectively,

energies -162.5

I of f o r m a t i o n of ~ Mg~Nz

kJ/mol

compared

and

-246.7

kJ/mol,

to -92.7

kJ/mol

for

formation of Li3 N at 400°C. FIGURE I Ionic conductivity of mixtures of Li2S and LiBr in various ratios. A "regular" variation is observed.

fore,

no

tendency

to

replace

binary or ternary compounds as anode material

Lithium has, Mg

or

AI

the

therein the

and may be employed

in practical galvanic cells.

B. Schoch et al. / New fast solid lithium ion conductors

532

Mg3N 2 and AIN

to form the

~-~cm -~

ternary c o m p o u n d s LiMgN and Li3AIN2,

Li3N

reacts

respective-

0.71 eV is found for Li3AIN 2. The higher conduc-

ly 11 . B o t h with

an

with

ternary l i t h i u m nitrJdes crystallize

antifluorite

are s t a t i s t i c a l l y dral

sites

radii

type

because

of

of Li + and M g ++

spectively)

structure.

distributed the

over very

LI

the

and Mg

tetrahe-

similar

ionic

(0.059 and 0.058 nm,

whereas

Li

and

of Li3N

(98%),

AI

are

re-

structurally

tivity

of

higher

Li3N

in

the

appropriate

were

annealed

pure

nitrogen.

helium

nitrogen 1100oc

(99%)

Pellets dry

box

were

because

of

the

inside

uniaxial

to

a

pressure.

and

N2

at

investigation

13 h a s

indicated

3 which

present

shows

study.

These data are included in

the r e s u l t s

A

obtained

conductivity

of

I

in the

6 . 2 x I 0 -7

400

300

200

I

I

I

I

than

for

open,

O

that

Li3A(N2 ~',

~ , ~ A=0.7' ev

1.0

°C

Li3N.

On

atomic

the

conductivity

d]-

other

arrangment

cationic

hand,

of

Li3N

decreases.

Some

lithium

compounds investi-

4. O R G A N I C A D D I T I O N COMPOUND L I T H I U M CONDUCTORS Early

investigations

formation

of

several

14 h a v e

addition

indicated

compounds

the

between

lithium halides and aleoholes or amines.

systems

of

has

several

confirmed

especially

lithium

halide-alcohol

the presence of a variety

observed

in the systems LiI-me-

and LiBr-ethanol.

halide

is very

to

[

I

1.4

1.8

the

It has

The

simple

p r e p a r a t i o n of the

and

liquid

by a d d i n g

alcohol

to be taken excessive

at

the

care of

room

lithium tempera-

the moisture,

h e a t i n g during the reac-

tion should be avoided. The

-4

600

gation.

however,

I

at

enthalpy of 2.0 eV. The

are stable with lithium are under

ture.

\~,

* E-2.0eV

I

~-Icm-~

3.

% %

~

I

pure

and the

ternary

compounds

O)

8.5xi0 -6

spaceous

disappears

thanol

.9, ° -2

is

an a c t i v a t i o n

in Fig.

d e c o m p o s i t i o n voltage of the t e r n a r y c o m p o u n d is

were

~A

is also obser-

of c r y s t a l l i n e phases. High ionic c o n d u c t i v i t i e s

E x

octa-

in view of the n o m i n a l l y

composition

The data are included

conductivity

Screening

T [°C] 800 6 0 0

(at

a much

higher c o n d u c t i v i t y and lower a c t i v a t i o n enthalpy for this compound.

conductivity

and shows

other

ionic c o n d u c t i v i t y of Li3AIN 2 has 12 early been r e p o r t e d by Roth et al. , but a very

Fig.

The

the

Li

the con-

interstitials

the ionic transport via intersti-

ved for LiMgN.

higher

of

If this increases

lithium

stoichiometric

under

Appreciable

recent

ideal

below

loss

be due to their

cy mechanism.

performed

higher temperatures.

al. 13 m a y

tials may be more favorable compared to a vacan-

temperatures

were

restricted

ratios

boats under

pressed

under

measurements

and

and AIN

stoichiometric

in covered m o l y b d e n u m

filled

Ae-impedanee

Mg3N2

et

content. of

hedral sites),

A high

Mixtures

°C and an a c t i v a t i o n enthalpy of

Yamane

centration

ordered.

(99%)

at 200

I

compounds

filled

2.2

10 3 [K 4]

T

dency vity

glove

bags.

measurements the melts

electrodes. diameter

and

prepared

in

nitrogen

The melts have a strong ten-

to undercool.

tion of FIGURE 3 Ionic c o n d u c t i v i t i e s of LiMgN and Li3AIN2. Literature data ~3 .for Li3AINz are i n d i c a t e d by the broken line.

were

Solid samples

were

prepared

between

Alternatively,

two

for conductiby

solidif~ca-

inert m o l y b d e n u m

cylinders of 10 mm in

I-4 mm in height were pressed from

the powders under pressures of 4 MPa. M o l y b d e n u m

B. Schoch et al. / N e w fast solid lithium ion conductors T [°C]

J

observed

25

60

-20

0

-40

for

the

Elemental

g

f

molten

o

I:

reaction,

-I

k~o ~'~

Io

T I.--

Lil - 4 CH30H EA=0.51 eV

°

I -2

lithium

LiI.4CH~ OH in

experimental various

'~o, %

o

system

LiBr-ethanol

at

room

solid

and

temperature.

i

v

533

in contact

did

contrast

to

batteries

cathodes,

not

pure

using

e.g.,

with

show

any

methanol.

lithium

MnO2

chemical Small

anodes

or TiS2,

easily prepared by immersing the electrodes the molten

\

cation.

\

-3

voltages

Y"x,

electrolyte

The currents corresponded

lithium concentration

and subsequent

are reversible

and

could

be

into

solidifi-

and the cell

to the values expected for cells.

REFERENCES I

I

3.5

4

I. A. Rabenau,

103 -I ~-[K]

Solid State Ionics 6 (1982) 277

2. J.H. Jackson and D.A. Young, Sol. 30 (1969) 1973

FIGURE 4 Arrhenius plot of the ionic conductivity of LiI.4 methanol. Melting is indicated by a steep increase in the conductivity.

3. I. Barin, O. Knacke, Thermochemical Properties of Inorganic Substances (Springer Verlag Berlin, 1973); -, and O. Kubaschewski, Supplement (Springer Verlag, Berlin, 1977) 4. W. Weppner and R.A. Huggins, lonics I (1980) 3

sheet

electrodes

pellets. the

A temperature

melting

plots

point

showed

lines

under

axis.

Melting

observed

the

in Fig. dence

springloaded

an

was

angle and

covered.

of

the

are

of the conductivity

measurements

compound

LiI.4CH30H

are shown

type temperature enthalpy

conductivity

depen-

of 0.6 eV is

is purely

ionic

and

one of the highest among all known solid lithium ion conductors. ty is similar

The room temperature to the value reported

the fast conducting crystallographic The compound

melts

in the crystalline ductivites

than

in

is apparently

phase. the

for Li3N

of

6. J.B. Wagner, Jr., Mater. 1691

Res. Bull.

15 (1980)

7. C. Wagner, in: Proc. Int. Comm. Electrochem. Thermodyn. and Kinetics (CITCE), Lindau 1955 (Butterworth, London, 1957) p. 361

8, R.A. Huggins,

Electrochim.

Acta 22 (1977) 773

9. B. Schoch et al., Z. anorg, (1984) 137 I0.C. Wagner, 1051

allg. Chem. 518

J. Phys. Chem. Solids 33 (1972)

11.R. Juza et al., Angew. Chem.

80 (1968) 373

in

12. W.L. Roth et al., NASA Rep. NAS 3-15692 (1972)

direction I. 13. B. Yamane et al., Solid State Ionics 15 (1985) 51

lower than

Even some higher case

J.

conductivi-

at 48±3 °C. The conductivity

of the glassy structure

5, W. Weppner, P. Hartwig, and A. Rabenau, Power Sources 6 (1981) 251

real

transitions

conductivity.

an activation The

straight

in the

addition

with

the

Solid State

impedance

by

45 ° against phase

changes

4. An Arrhenius

observed.

The

followed

other

by rapid

against

range from -40°C to above

semicircles

The results for

were

J. Phys. Chem.

methanol

conwere

14. F. Oschatz, Dissertation, Universit~t Jena, 1926; G. F. HHttig, Monatsh. f. Chemie 53/54 (1929) 299