Synthesis and ionic conductivity of lithium-conducting titanium phosphate solid electrolytes

Article abstract of DOI:10.1023/B:RJAC.0000044115.80809.57

Solid electrolytes were synthesized in the systems Li₂O-Al ₂O₃-TiO₂-P₂O₅ and Li ₂O-Al₂O₃-TiO₂-P₂O ₅-H₂O-H₂O₂. Their ionic conductivities were studied and compared. The possibility of obtaining a film of Li_1.3Al_0.3Ti_1.7(PO₄)₃ solid electrolyte on a sapphire substrate from an aqueous peroxide solution of a precursor was analyzed.

Full text of DOI:10.1023/B:RJAC.0000044115.80809.57

Russian Journal of Applied Chemistry, Vol. 77, No. 6, 2004, pp. 915 920. Translated from Zhurnal Prikladnoi Khimii, Vol. 77, No. 6, 2004,
pp. 929 934.
Original Russian Text Copyright
2004 by Kunshina, Gromov, Kuz’min, Seitenova, Lokshin, Kalinnikov.
APPLIED ELECTROCHEMISTRY
AND CORROSION PROTECTION OF METALS
Synthesis and Ionic Conductivity of Lithium-conducting
Titanium Phosphate Solid Electrolytes
G. B. Kunshina, O. G. Gromov, A. P. Kuz’min, E. B. Seitenova,
E. P. Lokshin, and V. T. Kalinnikov
Tananaev Institute of Chemistry and Technology of Rare Elements and Mineral Raw Materials,
Kola Scientific Center, Russian Academy of Sciences, Apatity, Russia
Received March 28, 2003; in final form, September 2003
Abstract Solid electrolytes were synthesized in the systems Li2O Al2O3 TiO2 P2O5 and Li2O Al2O3
TiO2 P2O5 H2O H2O2. Their ionic conductivities were studied and compared. The possibility of obtaining
a film of Li1.3Al0.3Ti1.7(PO4)3 solid electrolyte on a sapphire substrate from an aqueous peroxide solution
of a precursor was analyzed.
To be suitable for practical application in chemical
power cells or secondary batteries, a solid electrolyte
must primarily have the maximum ionic conductivity,
so that potential drop across the internal resistance
of a cell be much lower than its working voltage.
Presently, only few lithium-based solid electrolytes
with a room-temperature ionic conductivity on the
order of 10 Cm cm are known [1]. These are, in
the first place, lithium analogues of nasicons, com-
pounds based on double lithium and titanium phos-
phates, in which some Ti4+ cations are replaced with
In3+, Sc3+ or Al3+ cations.
the materials. These drawbacks are due to the mech-
anism of solid-state reactions, which occur originally
at the interface and then continue owing to the dif-
fusion of atoms across a layer of the products formed.
In the course of a reaction, the diffusion length in-
creases and the reaction rate decreases. The ceramic
process can commonly come to sufficiently full com-
pletion only in the case of an intermediate multiple
grinding of the reaction mass. But even then, the re-
quired chemical and structural homogeneity of the
product cannot be always achieved. To overcome the
above difficulties, various modifications of the ceram-
ic method, based on a number of procedures for pre-
liminary homogenization down to the molecular level
of the solid-phase reaction mixture, have been sug-
gested. These are coprecipitation of components from
solutions, drying of solutions by spraying, pulveriza-
tion of the solutions into liquid nitrogen (cryochem-
ical method), or sol gel procedure. The sol gel meth-
od shows promise for synthesis of lithium-based solid
electrolytes [4].
3
1
An important requirement to the solid electrolytes
of lithium power cells is their high chemical stability
when in contact with cathode and anode materials.
In this case, it is necessary to take into account that
metallic lithium or lithium alloys used as anode are
rather chemically active. The known solid electrolytes
based on double lithium and titanium phosphates are
chemically unstable at elevated temperatures because
of the interaction of metallic lithium with titanium
phosphate [2]. Hence follows that, to enhance the
chemical stability of this type of solid electrolytes,
it is necessary to decrease the content of titanium and
phosphate groups in them.
The goal of this study was to search for ways to
raise the ionic conductivity and chemical stability
of phosphate lithium-conducting solid electrolytes
in the solid-phase and sol gel syntheses.
Previously [3], Li1.3Al0.3Ti1.7(PO4)3 solid electro-
lytes have been synthesized by the ceramic method.
However, this technique has some drawbacks limiting
its application in the cases of increased requirements
to the stability and reproducibility of properties of
EXPERIMENTAL
As starting reagents were used Li2CO3 (special
purity); LiNO3 (analytically pure); Al(NO)3 9H2O,
1070-4272/04/7706-0915 2004 MAIK Nauka/Interperiodica

916
KUNSHINA et al.
digital LCR-meter at a frequency of 1 MHz in a cell
with graphite electrodes on several pellets of different
heights. For samples of the same composition, the
25
values were reproduced to within 5%.
The electronic conductivity
was determined by
the polarization Wagner methode in a cell with irre-
versible graphite electrodes. A voltage lower than the
voltage of electrolyte decomposition was applied to
the measuring cell and the current in the circuit was
measured with a U5-11 charge amplifier [6].
The phase composition of the products synthesized
was studied by X-ray phase analysis (DRON-2 dif-
fractometer, CuK radiation, graphite monochroma-
tor).
Fig. 1. Ionic conductivity
of double lithium and tita-
25
nium phosphates vs. Li O : TiO molar ratio N.
2
2
The base component of the known lithium-con-
ducting solid electrolyte Li1.3Al0.3Ti1.7(PO4)3 is dou-
ble phosphate LiTi2(PO4)3 with the molar ratio Li2O :
TiO2 = 1 : 4. No data for double lithium and titani-
um(IV) phosphates with other molar ratios are avail-
able.
Al2O3, and (NH4)2HPO4 (all chemically pure); TiO2
(special purity); and (NH4)2TiO(SO4)2 H2O (chem-
ically pure). Freshly precipitated titanium(IV) hydrox-
ide was obtained by dissolving a weighed portion
of titanium(IV) oxide of the anatase modification in
hydrofluoric acid. The resulting acid fluoride solution
was heated to 50 60 C and added in small portions
to a 25% solution of ammonium hydroxide under
vigorous agitation. The precipitated amorphous tita-
nium(IV) hydroxide was washed with a 5% NH4OH
solution to remove fluoride ions by repulping. The
The synthesis of double lithium and titanium(IV)
phosphates and the measurement of their ionic con-
ductivity were performed at the Li2O : TiO2 ratio in
the range from 1 : 4 to 4 : 1. These compounds were
synthesized by the ceramic method [3] at the temper-
ature of the final sintering stage equal to 800 1000 C,
depending on composition. The results obtained in
1
content of the F ions was controlled potentiometri-
cally using a Kh-F-001 fluoride-selective electrode.
a study of
of the samples obtained are presented
1
25
The content of the F ions in the washing water from
in Fig. 1. It can be seen that the optimal composition
is obtained at Li2O : TiO2 = 1 : 1 and corresponds to
chemical formula Li2Ti(PO4)2. The generally accepted
composition LiTi2(PO4)3 is unremarkable.
1
the final repulping was 10 mg l and less.
The method for determining the ionic conductivity
(at 25 C) of the samples of lithium-conducting
25
solid electrolytes has been described in detail previ-
ously [5]. The measurements were done on an E7-12
To study the influence exerted on the ionic conduc-
tivity by the Ti4+ Al3+ +Li+ heterocharge substitution,
experiments on the synthesis of Li2 + xAlxTi1 x(PO4)2
solid solutions were performed. The results pres-
ented in Table 1 show that such a substitution re-
sults in an increase in the ionic conductivity only at
x = 0.1.
Table 1. Ionic conductivity of Li2 + x AlxTi1 x (PO4)2 and
Li2 3x AlxTi1 x (PO4)2 solid solutions
1
Solution
(I)
Composition
25, Cm cm
Because the composition Li2Ti(PO4)2 is charac-
terized by relatively high concentration of lithium
ions, it was appropriate to perform the heterocharge
5
Li2Ti(PO4)2
2.7 10
1.3 10
4
Li2.1Al0.1Ti0.9(PO4)2
Li2.2Al0.2Ti0.8(PO4)2
Li2.3Al0.3Ti0.7(PO4)2
Li2Ti(PO4)2
Li1.7Al0.1Ti(PO4)2
Li1.4Al0.2Ti(PO4)2
Li1.1Al0.3Ti(PO4)2
Li0.8Al0.4Ti(PO4)2
5
substitution 3Li+
Al3+. The data of Table 1 show
3
10
5
5
4
4
4
4
2.3 10
2.7 10
2.7 10
5.4 10
5.6 10
2.5 10
that this type of heterocharge substitution consider-
ably increases the ionic conductivity in the range
(II)
0.1
x
0.4, with the ionic conductivity of the com-
position Li1.1Al0.3Ti(PO4)2 being the highest.
The results of experiments with mixed heterocharge
substitution, i.e, with joint occurrence of Ti4+ Al3+ +
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 77 No. 6 2004

SYNTHESIS AND IONIC CONDUCTIVITY OF LITHIUM-CONDUCTING SOLID ELECTROLYTES 917
Li+ and 3Li+
Al3+ processes, are listed in Table 2.
Table 2. Ionic conductivity of Li2Ti(PO4)2-based solid
solutions with a mixed heterocharge substitution
It can be seen that the composition Li1.4Al0.2Ti(PO4)2,
in which the Ti : Li and PO4 ratios are 0.71 and
1.43, respectively, is the most promising from the
standpoint of chemical stability. At the same time, for
the generally accepted composition Li1.3Al0.3Ti(PO4)2,
these ratios are 1.3 and 2.31.
3
1
Composition
25, Cm cm
4
Li0.9Al0.1Ti1.2(PO4)2
Li1.0Al0.2Ti1.1(PO4)2
Li1.1Al0.3Ti(PO4)2
Li1.2Al0.4Ti0.9(PO4)2
Li1.3Al0.1Ti1.1(PO4)2
Li1.4Al0.2Ti(PO4)2
Li1.5Al0.3Ti0.9(PO4)2
Li1.8Al0.2Ti0.9(PO4)2
1.4 10
6.2 10
5.6 10
2.4 10
4.2 10
5.4 10
1.4 10
1.1 10
4
4
The room-temperature electronic conductivity of
Li1.4Al0.2Ti(PO4)2 is 3 10 Cm cm . Its ionic con-
ductivity linearly increases in the range 25 100 C
4
8
1
4
4
4
4
1
from 5.4 10 to 8.1 10 Cm cm .
4
4
to pH 2 3. Otherwise, crystalline quick-settling pre-
cipitate appeared in the solution instead of a suspen-
sion. The suspension obtained was poured onto a fluo-
roplastic pan, evaporated, and dried at 100 110 C
for 8 h. As a result, a precursor of the solid electrolyte
was formed as a loose bulky powder.
To obtain the final product, the precursor was cal-
cined. The temperature mode and calcination time
were determined from the results of thermal and X-ray
phase analyses. According to the thermal analysis,
the weak endothermic effects exhibited by the elec-
trolyte precursor (Fig. 2) at temperatures of up to
158 C are due to removal of water. The exothermic
effect at 254 293 C is associated with the interaction
of LiNO3 and (NH4)2HPO4, with evolution of nitro-
gen oxides and NH3 (NH4)2TiO(SO4)2 decomposes
within the range 400 700 C. At higher temperatures,
no thermal effects or mass loss was observed. There-
Scheme of the sol gel process.
The sol gel synthesis of lithium-conducting phos-
phate solid electrolytes was performed using 1 M
aqueous solutions of the starting reagents, LiNO3,
Al(NO3)3, (NH4)2HPO4, and (NH4)2TiO(SO4)2 H2O,
taken in stoichiometric amounts in accordance with
the formula Li1.3Al0.3Ti1.7(PO4)3. The process by
which the solid electrolyte was synthesized is illus-
trated by the scheme. A neutral solution of LiNO3
(pH 6) and a weakly alkaline solution of (NH4)2HPO4
(pH 8) were poured together, and the acidity of the
resulting solution was adjusted to pH 2 3 with a
20% solution of HNO3. Then, Al(NO3)3 (pH 3) and
(NH4)2TiO(SO4)2 (pH 1) solutions were successively
poured-in under vigorous agitation. As a result, a
white suspension was formed. The necessary condi-
tion was that the pH value was preliminarily adjusted
Fig 2. Thermogram for a precursor of lithium-conducting
solid electrolyte Li Al Ti (PO ) . (T) Temperature.
1.3 0.3 1.7
4 3
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 77 No. 6 2004

918
KUNSHINA et al.
loose, and easily disintegrating) was compacted into
pellets 12 mm in diameter and 2 3 mm thick and sin-
tered at 800 1000 C. The sintered pellets frequently
separated into layers, cracked, and deformed. To im-
prove sintering and avoid deformation of the pellets,
a C2H5OH binder was introduced into the powder
before compaction. The diffraction patterns of the
electrolyte samples sintered at T > 800 C contain only
reflections associated with LiTi2(PO4)3 (Fig. 3a). As
follows from Table 3, the highest ionic conductivity
4
1
= 5 10 Cm cm is observed for the electrolyte
25
sample sintered at 1000 C for a long time (7h).
In preliminary studies, we established that freshly
precipitated titanium(IV) hydroxide satisfactorily dis-
solves in the form of a peroxide complex in an acid
medium. To synthesize lithium-conducting solid elec-
trolyte Li1.3Al0.3Ti1.7(PO4)3, a freshly precipitated ti-
tanium(IV) hydroxide was dissolved in initial exper-
iments in a mixture containing 40 vol % H3PO4 (acid
concentration 85 wt %) and 60 vol % H2O2 (peroxide
concentration 30 wt %). After stoichiometric amounts
of aqueous solutions of LiNO3 and Al(NO3)3 were
added to the resulting titanium(IV) peroxide solution,
a transparent solution (pH 0.7) of intense ruby color
was formed. After evaporation of this solution at 150
160 C and subsequent drying of the precipitate at
300 C, a white powder was formed. The powder
was compacted into pellets and sintered at 900 C for
1 h, with a mass loss of 35%. The sintered pellets
were porous and could be easily broken. To obtain
high-density pellets, sintering was performed at
1000 C for 1 h. It was found that the ionic conduc-
7
1
tivity is about 1 10 Cm cm , i.e., is lower by
three orders of magnitude than the predicted value.
Additional sintering at 1200 C did not affect . Ac-
cording to XPA, the diffraction pattern of the product
synthesized contains no reflections corresponding to
Li1.3Al0.3Ti1.7(PO4)3 (Fig. 3b), with the main reflec-
tions belonging to titanium diphosphate TiP2O7. Ap-
parently, titanium(IV) polyphosphates, which do not
decompose even at high temperature, are formed ow-
ing to excess content of H3PO4 in the initial solution.
In this connection, HNO3 was used for obtaining
an acid medium in dissolution of a freshly precipi-
tated titanium(IV) hydroxide. The freshly precipitated
titanium(IV) hydroxide was dissolved in a mixture
containing 70 vol % HNO3 (65 wt % concentration)
and 30 vol % H2O2 (30 wt % concentration). To the
resulting solution, stoichiometric amounts of aqueous
solutions of LiNO3, Al(NO3)3, and H3PO4 were added
under agitation. The resulting aqueous peroxide solu-
tion of ruby color was transparent and contained 160
170 g l 1 of the final product. Evaporating the solution
Fig. 3. Diffraction patterns of (a) lithium-conducting solid
electrolyte Li Al Ti (PO ) , (b) product of a synthesis
1.3 0.3 1.7
4 3
of solid electrolyte with a 1.5-fold excess of H PO , and
3
4
(c) Li Al Ti (PO ) film on a sapphire substrate.
1.3 0.3 1.7
4 3
(1) LiTi (PO ) and (2) TiP O .
2
4 3
2 7
fore, the temperature was raised stepwise to 700 C.
The precursor powder was kept at 250 C for 1 h (till
gas evolution stopped) in the first stage and at 700 C
for 2 h in the second. The product obtained (bulky,
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 77 No. 6 2004

SYNTHESIS AND IONIC CONDUCTIVITY OF LITHIUM-CONDUCTING SOLID ELECTROLYTES 919
in a desiccator at 150 160 C yielded a white powder,
which was sintered at 900 C for 1 h with a mass loss
of 31%. The powder was compacted into pellets and
sintered at 1000 C for 1 h. The ionic conductivity of
Table 3. Ionic conductivity of Li1.3Al0.3Ti1.7(PO4)3 at
different conditions of pellet sintering
1
Tsin
,
C
sin, h
htab, cm
25, Cm cm
4
1
the sintered pellets was 6 10 Cm cm , which co-
incides with the conductivity of Li1.3Al0.3Ti1.7(PO4)3
samples obtained by the solid-phase synthesis. The elec-
5
800
900
2
4
2
7
2
7
0.227
0.181
0.130
0.118
0.428
0.427
4
10
4
4
4
4
4
1.1 10
3.9 10
3.8 10
3.8 10
5.0 10
8
1
tronic conductivity did not exceed 2 10 Cm cm .
The scheme described was used to synthesize
Li2Ti(PO4)2 solid electrolyte and a number of elec-
trolytes on its basis. The ionic conductivities of the
samples synthesized are listed in Table 4.
1000
Table 4. Ionic conductivity of Li2Ti(PO4)2-based solid
electrolytes
It can be seen that the ionic conductivities of the
samples obtained by two methods are virtually the
same.
*
1
Composition
,
Cm cm
25
The main advantage of the method developed for
preparing stable aqueous peroxide solutions in the
Li2O Al2O3 TiO2 P2O5 H2O H2O2 system consists
in the possibility of using such solutions for obtaining
thin films of solid electrolytes. Aqueous peroxide so-
lution of a precursor of the Li1.3Al0.3Ti1.7(PO4)3 elec-
trolyte, which contained glycerol as a film-forming
agent, was used to deposit films onto substrates by
dipping method.
4
4 *
Li1.1Al0.3Ti(PO4)2
Li1.4Al0.2Ti(PO4)2
Li1.6Al0.2Ti(PO4)2
Li2Ti(PO4)2
4.8 10 (5.6 10 )
4
4
4.5 10 (5.4 10 )
4
2
1
10
4
4
10 (1 10 )
*
The
values for samples obtained by solid-phase method
25
are given in parentheses.
is observed for the composition Li2Ti(PO4)2. The
A solution containing (per 100 ml) 12 g of a pre-
cursor of the Li1.3Al0.3Ti1.7(PO4)3 solid electrolyte and
5 ml of glycerol was stable (i.e., retained transparency
and homogeneity) for seven days. To apply a film, the
substrate was dipped into a solution and then with-
Li1.4Al0.2Ti(PO4)2 solid electrolyte with
=
5.4 10 Cm cm and the Ti : Li and PO34 : L2i5mo-
lar ratios of 0.71 and 1.43, respectively, was obtained
upon heterocharge substitution of some Li+ ions for
Al3+ ions in this compound. The same ratios for the
generally accepted composition Li1.3Al0.3Ti1.7(PO4)3
are 1.3 and 2.31.
4
1
1
drawn at a rate of 0.6 cm min , dried in air, annealed
at 450 C for 15 min, and cooled to room temperature.
Then the solution was applied to the substrate again,
only with the sample annealed at 850 C for 30 min
this time. The existence of the film was confirmed
by visual observation under a Neophot-2 microscope
(magnification 200). An X-ray phase analysis showed
that four relatively weak reflections (6.04, 4.26, 3.63,
and 3.02 ), which are characteristic of a powder-
like electrolyte, are present in the diffraction pattern
of the film obtained (Fig. 3c). The study performed
demonstrated that dipping is a suitable method for
obtaining on appropriate substrates films of lithium-
conducting titanium phosphate solid electrolytes,
which are promising for fabrication of thin-film chem-
ical power cells.
(2) A method for obtaining stable aqueous perox-
ide solutions of precursors of solid electrolytes in the
Li2O Al2O3 TiO2 P2O5 H2O H2O2 system was de-
veloped. It was established that the solid electrolytes
obtained from these solutions compare well in the
ionic conductivity with electrolytes synthesized by
the solid-phase method.
(3) It was shown that aqueous peroxide solutions
of precursors are suitable for obtaining films of lith-
ium-conducting titanium phosphate solid electrolytes
on appropriate supports, which are promising for fab-
rication of thin-film chemical power cells.
ACKNOWLEDGMENTS
CONCLUSIONS
The study was financially supported in part by the
New Principles and Methods of the Development and
Directional Synthesis of Substances with Prescribed
Properties Integrated Program of the Presidium of
the Russian Academy of Sciences.
(1) A study of the ionic conductivity of solid
electrolytes obtained by the solid-phase method in
the Li3PO4 Ti3(PO4)4 system established that the
5
1
maximum conductivity (
= 2.7 10 Cm cm )
25
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KUNSHINA et al.
5. Kunshina, G.B., Kuz’min, A.P., and Gromov, O.G.,
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