SOLID
STATE
Solid State Ionics 51 (1992) 79-83
North-Holland
IONICS
Lithium insertion in vanadyl phosphate
R. Pozas, S. Maduefio, S. Bruque, L. Moreno-Real, M. Martinez-Lara,
C. Criado a and J. R a m o s - B a r r a d o a
Departamento de Quimica Inorgttnica and Departamento de FisicaAplicada a, Universidad de Mddaga,
Apartado 59, 29071 Malaga, Spain
Received 7 October 199!; accepted for publication 20 November 1991
The reaction between LiNO3 and VOPO4.2H20 in acetone medium takes place, besides a redox process, by an acid-basic
reaction. The material obtained, L i l . 6 V O P O 4 " H 2 0 , is a non-homogeneoussolid, which is converted to a crystalline solid if annealed at 450°C. The existence of lithium ion within the intracrystalline spaces of lithium intercalate as well as the reduction of
partial V(v) to V(iv) leads to a mixed ionic-electronicconductor. This fact is confirmedby the impedancespectroscopystudy.
1. Introduction
Much progress in vanadyl phosphate chemistry and
technology has been made in recent years. These
phosphates are of interest for their catalytic properties in the chemical industry for the synthesis of
maleic anhydride from n-butane or n-butene [ 1-3 ].
The structure of a-VOPO4- 2H20 (VP) consists of
infinite sheets of distorted VO6 octahedra and PO4
tetrahedra linked by shared oxygen atoms. Shared
water molecules link these layers together [4]. Acidbase and redox reactions can lead to very different
"vanadium phosphates". Because of the existence of
H30 + ions in the lattice of VOPO4-2H20 [ 5 ] it presents Bronsted acidity. Also, when the vanadium
atom acts as electron acceptor a Lewis acidity is observed [6,7]. Thus, the solid oxovanadium(v)
phosphate dihydrate is therefore considered as the
parent Bronsted acid (through the equilibrium
V O P O 4 " 2 H 2 0 - , H O - V = O ( H 3 0 ) P O 4 ) of salts like
AHVPO6 ( A = N H + , K +, Rb +, Cs +, T1+ ) [8-10].
The redox intercalation reactions in VP provoke
the lowering in the oxidation state on the vanadium,
and so the existence of the mixed valence compounds which could have a possible electronic mobility and conductivity. At the same time, the inserted cations in order to charge compensating, could
have a mobility if these are sufficiently small and
cavities or channels are available in the structure.
Elsevier SciencePublishers B.V.
Solid mixed conductors of ions and electrons, besides being an interesting physical system, are particularly useful as solid solution electrodes in secondary batteries. The ionic and electronic charges
can be introduced by thermal excitation, deviation
from stoichiometry and doping.
The aim of this work is to test a new way to prepare mixed ion-electron conductors as well as to
study their electrical properties and the physicochemical characterization of the solids derived from
lithium intercalation in VP.
2. Experimental
Crystalline a-VOPOa'2H20 (VP) was prepared
as described previously [ 10]. Insertion of lithium
into this solid was made by contacting VP with
LiNO3"H20 in acetone medium. The reaction conditions were the following: molar ratio L i / V P = 8 ,
temperature 60°C, contact time solid/solution seven
days. We consider the end of the reaction when 7.40
reflection in the X-ray diffractogram, characteristic of VP, has disappeared.
The final suspension from the above reaction was
then filtered, and the solid was washed with acetone
until excess nitrate was eliminated. The product was
air dried.
The chemical composition of the lithium deriva-
80
R. Pozas et al. / Lithium insertion in vanadyl phosphate
tive was determined by dissolving the samples in hot
sulfuric acid. Phosphorous content was analyzed by
colorimetric method as the molibdophosphate complex. Lithium was determined by emission spectroscopy, and total vanadium and V (IV) were analyzed
by redox titration. Water content was measured by
thermogravimetric analysis.
The material was investigated by X-ray diffraction
(Siemens D501), UV-VIS-NIR spectroscopy (Shimadzu 3100), IR spectroscopy (Perkin Elmer 883),
D T A - T G (Rigaku Thermoflex) and impedance
spectroscopy (FRA Solartron 1255, between 1 Hz
and 12 MHz and temperatures from 673 K to 333
K).
The pellets, 13 m m in diameter and 1 m m thickness, were prepared by pressing the material at 6 MPa
and then sintered at 723 K in N2, were then coated
with gold to make conductivity measurements.
The reaction between LiNO3 and VP in acetone
medium provokes a change in the lattice of the original solid as can be seen in fig. 1. The X-ray powder
diffractogram of LiVP exhibits more and wider reflections than the pristine solid. Moreover, it has not
been possible to assign Miller indices at all the reflection peaks within any crystalline system. For this
reason, we could infer that in this solid are present
more than one crystalline phases, probably due to a
non-homogeneous lithium diffusion through the
crystallites of the original lattice. Nevertheless, when
the sample is annealed at 450°C in N 2 atmosphere
3. Results and discussion
3.1. Characterization a n d structural properties
The chemical analysis of the lithium vanadium
phosphate gives the following empirical formula
Li 1.6VOPO4 • H 2 0 ( L i V P ) . The potentiometric titration of vanadium in this material indicates a part of
vanadium as V ( I V ) (25%) because of the partial reduction during the insertion process. The easiness of
reduction that the VP presents in intercalation reactions when there is water, even as traces, is well
known. This change in the oxidation state of vanadium in VP is associated with a colour modification
in solid from yellow to brown. A consequence of these
V ( I V ) species in the solid such as charge defects are
created in its lattice which are balanced by the close
presence of lithium ions.
The lithium insertion within VOPO4-2H20 takes
place, besides a redox process, by an acid-base reaction. This is inferred from its empirical formula
since the L i / V molar ratio is superior to 1. The water
molecule coordinated with the vanadium atom of the
pristine compound exhibits a remarkable acid behaviour [ 10 ]. This acidity is responsible for the lithium excess into the final product.
Acid-base reaction types have also been induced
in isomorphous solid of the VP [ 12 ].
i
,B/
AA
31
35
39
43
47
51
55
L~Bdegrees
Fig. 1. X-ray diffraction patterns of: (A) VP; (B) LiVP and (C)
LiVP annealed at 450 °C, (*) orthorhombic phase.
R. Pozas et al. /Lithium insertion in vanadyl phosphate
(sample LiVP-450 hereafter) to prevent the total
oxidation of the V (IV) in the LiVP, produces a solid
with a majority phase which presents a high crystallinity a n d can be indexed in the o r t h o r h o m b i c system with a good merit figure ( M 2 o = 2 0 ) using the
T R E O R program [ 13 ]. The Miller indices of the reflections as well as the unit-cell parameter for LiVP450 are shown in table 1. The m i n o r phase which coexists together with orthorhombic phase is not possible to be indexed since there are few reflection lines
in the powder X-ray pattern a n d these have a weak
intensity (see fig. l ).
81
tions are shaper a n d its assignments are given in table 2.
U V - V I S - N I R diffuse reflectance spectra of LiVP
at room temperature and after being heated at 450°C
in N 2 are shown in fig. 2. Both solids display a great
background with an almost constant absorption in
the wavelength zone measured. This can be tentatively attributed to the creation of bandgap states in
the electronic structure of these solids. Spectral features indicate that new transitions are allowed
through lithium insertion into the VP lattice: charTable 2
Infrared spectrum data of LiVP-450.
3.2. Optical properties
IR spectrum of l i t h i u m derivative ( L i V P ) shows
a great absorption o n the 1000 c m - t region with several bands, a m o n g these, the b a n d s at 1045 a n d 1085
c m - ~are conspicuous. The absorption-width a n d its
multiplicities of lines indicate the existence of various e n v i r o n m e n t s with different symmetry for the
PO4 groups in the sample. This suggests the coexistence of several crystalline phases in the solid that
is corroborated by the X-ray powder pattern. W h e n
this solid is annealed in N 2 at 4 5 0 ° C the IR absorp-
~,(crn- ~)
Assignment
Intensity
1153
8 POH
m
1110
1007
897
616
vas PO4
~ s PO4
VV-OH
8POH
out of plane
8as OPO
8 s OPO
m
S
m
s
510
456
m
m
Table 1
X-ray powder diffraction data for orthorhombic phase of LiVP450 a)
h kl
dou, (l~)
d~,lc (l~)
I/Io
200
2 10
10 1
0 11
111
220
2 1 1, 3 1 0
02 1
12 1
311
13 1
32 1
40 1, 1 02
3 3 0, 2 3 1
5.1642
4.4247
4.2319
4.0809
3.7951
3.2980
3.2015
3.1429
3.0094
2.6315
2.3661
2.3221
2.2571
2.1998
1.9141
1.7247
1.6996
5.1693
4.4287
4.2306
4.0797
3.7950
3.3025
3.2025
3.1500
3.0133
2.6327
2.3704
2.3251
2.2576
2.2017
1.9141
1.7258
1.7011
17
100
37
15
49
44
17
14
56
49
28
16
23
39
12
15
23
14 1
402
232
*) Orthorhombic: a= 10.330(8) ,/~,b=8.579(3) ,~, c=4.632(2)
/k, V=410.61 ,~3.
A
190
400
600
HAVE
800
1000
L E NGTH ( n m )
Fig. 2. UV-VIS diffuse reflectance of: (A) LiVP and (B) LiVP
annealed at 450°C.
82
R. Pozas et al. / Lithium insertion in vanadylphosphate
acteristic profile of small polaron hopping phenomenon is evident between 600 and 1000 n m [ 14 ].
A weak absorption can be detected about 800 n m
over the mentioned polaron background corresponding to the 2E2,--2B2 electronic transition which
is typical of V ( I V ) compounds with V=O groups.
This absorption is lower, almost inappreciable, for
the solid heated at 450°C.
In the zone between 200 and 600 nm both solids,
LiVP and LiVP-450, exhibit a wide band with a great
intensity characteristic o f a L ~ M charge transfer.
This band for the lithium derivatives is wider than
the corresponding one in the VP. This can be explained if it assumed that the lithium insertion in the
lattice provokes a disturbance around the [VO6 ] octahedra creating different vanadium coordination
geometries as well as v a n a d i u m oxidation states.
3.3. Electrical properties
Fig. 3 shows the Nyquist plot o f LiVP-450. The
equivalent circuit for this c o m p o u n d was assumed to
be a parallel combination o f conductance G and a
constant phase element CPE, [ Z = A ( j o g ) ' ] . Table 3
gives its conductivity data. The chemical composition o f LiVP-450 and the Nyquist plots at different
temperatures suggest a mixed conductivity. We have
measured the electrical conductivity by the usual dc
four-probes method, in the same temperature range
( 6 7 3 - 3 3 3 K). A fixed direct current density (J) was
applied between the current electrodes. The voltage
was measured as a function o f time after switch-on.
Initially both ions and electrons flow so that the volt800
700
600
Table 3
Electrical conductivity data for LiVP-450.
T
(K)
trX 10 - 3
(s/cm)
~
(°)
--n
ti
-t~
333
348
361
380
402
425
459
508
568
673
0.100
0.122
0.177
0.490
0.547
0.867
1.92
4.29
6.81
16.0
3.53
2.39
2.12
2.06
2.00
1.93
1.61
1.43
1.12
0.980
0.910
0.973
0.976
0.977
0.978
0.981
0.982
0.984
0.987
0.989
0.910
0.918
0.892
0.890
0.872
0.862
0.854
0.797
0.770
0.719
0.0900
0.0820
0.108
0.110
0.128
0.138
0.146
0.203
0.230
0.281
li, te: ionic and electronic transport numbers; O: depressed angle
of Nyquist semicircle; n: from Z=A (jto)'.
age at t = 0 , Vo=Jy/a, where y is the distance between probes and a=tri+ae is the sum o f the ionic
and electronic contribution. With the time a concentration gradient grows up because the ions are
blocked at the current electrode and eventually the
electrochemical potential gradient drops to zero, so
the ions cease to flow. At this point V= V~=Jy/tre.
F r o m these two voltages, Vo and Voo, ai and tre can
be determined.
From the above results, the electrical conduction
in LiVP-450 is predominantly ionic. The activation
energy of 33 k J / m o l e for ionic conduction, calculated from the Arrhenius plot describing the hopping
model, means that lithium ions are the carrier species.
The electronic conductivity is always lower than
the ionic conductivity and it increases with the temperature; extrapolation of the data indicates that exclusive ionic conduction is likely to be observed in
the phosphate and only below 193 K. The electronic
component can arise from the coexistence o f V ( V )
and V (IV) in the solid.
500
400
o o °
300
•
o
•
o
°
•
e
• •
•
4. Conclusions
°
""
o~
200
100
iI
0
0
100
co%
. . . .
200
I . . . .
I . . . .
300
I . . . .
400
I . . . .
500
\
t.,,
600
-11
700
z(~)
Fig. 3. Nyquist plot of LiVP-450 at 530 K.
800
The insertion o f lithium ions into the intracrystalline space of V O P O 4 - 2 H 2 0 is a complex process
that involves a redox and the acid-base reactions. The
annealed in inert atmosphere leads to a crystalline
material in which exist the vanadium atoms in mixed
oxidation states and mobile Li ÷ ions. This solid ma-
R. Pozas et al. / Lithium insertion in vanadyl phosphate
terial
exhibits
electronic
and
mainly
ionic
conductivity.
T h e e x i s t e n c e o f a great b a c k g r o u n d a b s o r p t i o n in
the visible a n d n e a r i n f r a r e d r e g i o n s is a t t r i b u t e d to
t h e c r e a t i o n o f b a n d g a p states in the e l e c t r o n i c structure o f this m a t e r i a l . T h i s fact is a s s o c i a t e d to a c h a r acteristic spectral p r o f i l e o f small p o l a r o n h o p p i n g
phenomenon.
Acknowledgments
We wish to t h a n k C I C Y T f r o m M i n i s t e r i o de Edu c a c i 6 n y C i e n c i a de E s p a h a ( P r o j e c t M A T 90-298 )
a n d J u n t a de A n d a l u c i a ( R e s e a r c h g r o u p 6 0 2 7 ) for
financial support.
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