A thermodynamic study of electrochemical lithium insertion into vanadium pentoxide

A THERMODYNAMIC LITHIUM INSERTION

STUDY OF ELECTROCHEMICAL INTO VANADIUM PENTOXIDE

J. P. PEREIRA-RAMOS* and R. MESSINA Laboratoire d’Electrochimie, Catalyse et Synth&x Organique, CNRS (UM no 28), 2 rue Henri Dunant, 94320 Thiais, France C. PIOLET and J. DEVYNCK Laboratoire d’Electrochimie Analytique et Appliquke (UA 216) ENSCP, 11 rue Pierre et Marie Curie, 7523 Paris 05, France

1

(Received 8 October 1987) Abstract-The thermodynamics of the lithium insertion process into V205 to form Li,V,O, bronzes has been studied by electrochemical titration in galvanic cells reversible with respect to lithium. The partial molar quantities AG,, x,, AHLi,x and ASLi xj were obtained from e.m.f.-temperature measurements performed in molten dimethy Isulfone (+- - 120-l!O”C). The existence of two ordering processes was demonstrated to take place in the lithium vanadium bronze at x = 0.25/0.30 and x = 1.

thermodynamic study of lithium diffusion in V,O, 3. Li 1 +,VJ08 and (Mo~.~V~.,)~O~, has already been published[17-191. Very recently a comprehensive thermodynamic study of electrochemical lithium insertion into /I-LiXV205 and Li,V2_,M,05 (M = MO or W) bronzes[2&21] over a large range temperature has completed previous data on silver intercalation into P-Ag,V,O,[22]. Nevertheless such a study of the orthorhombic vanadium pentoxide, has to date not been performed. Thus we undertook an electrochemical investigation on the thermodynamics of electroformed Li,V,05 bronzes. The aim of our study was to determine the partial molar thermodynamic quantities characteristic of lithium insertion into Li,V,05 from potentiometric measurements. The work has been performed in molten dimethylsulfone (DMSO*) in which chemical electrochemical are and kinetics strongly enhanced[23-241.

INTRODUCTION In recent years considerable attention has been paid to the chemical and physical properties of insertion compounds, because of their potential application as cathode materials in secondary battery systems[l-31. Two kinds of materials have been mainly investigated. The first consists of the group IV, V and VI B transition metal dichalcogenides[4] and the second of transition metal oxides[5-6]. In spite of a great deal of available data about these compounds, few thermodynamic studies have been performed on the electrochemical insertion of lithium or other elements. Among the transition metal dichalcogenides, the Li,TiSz system has given rise to different models in order to explain the variation of the chemical potential of the alkali metal in the host structure[7-121. Nevertheless, the determination of partial thermodynamic functions has been restricted to lithium and silver intercalation into TiS,[13-141 and to copper insertion in Chevrel’s phases MO&_, (0 < x < 3.5)[15-161. As far as the transition metal oxides are concerned, the vanadium oxide-based compounds are known as the best cathodic material in lithium batteries. A

EXPERIMENTAL The thermodynamic study was carried out by performina e.m.f.-temperature measurements in reversible g&anic cells: _

Li,V,O,/l

mol kg-’

LiClO,/Li/l

mol kg-’

DMSO, working

electrode

l-*e.mf.A *Author to whom correspondence should be addressed. 1003

LiClO,/Pt DMSOl

reference

electrode

...7~~~

J. P. PEREIRA-RAMOS er al.

1004

Dimethylsulfone (DMSO,) (Janssen) was first recrystallized in water and then twice from absolute methanol, air dried at 90°C for 48 h and sublimated under reduced pressure (2 mm Hg) at 100°C. It was then conserved in an argon glove box. Under these conditions, the water concentration did not exceed 5 x 1O-3 mol kg-‘. Propylene carbonate (twice distilled) was obtained from Fluka and used as received. Anhydrous lithium perchlorate was dried under vacuum at 200°C for 12 h. The electrolytes were prepared under a purified argon atmosphere and the 1 mol kg-’ LiCIO,/DMSO, electrolyte give rise to a high conductivity 3 x lo-’ 0-l cm-’ at 150°C. The working electrode consisted of a thin platinum grid 1 cm in diameter, on which vanadium pentoxide powder (K.K. Laboratories) mixed with graphite (90 % (w/w)) was pressed. A lithium wire and a platinum wire immersed in separate compartments containing a 1 mol kg’ or 1 M LiC104 solution acted as reference and auxiliary electrode respectively. Pure metallic lithium has been proven to be rather stable in this medium and has already been employed as a reference electrode. A thin porosity frit was used to prevent any diffusion of Li+ ions. The working electrode composition was changed by coulometric titration. Reaching equilibrium after coulometric titration required a few hours at constant temperature in order to obtain a uniform Li+ ion distribution throughout the working electrode. Once equilibrium was reached, however, alteration to the e.m.f. equilibrium rapidly followed any change in thereafter (- 30 min). introduced temperature Equilibrium was considered to have been reached when the open circuit voltage remained stable (+- 1 mV) for 12 h., and the e.m.f.-T measurements were completely reproducible after an increase or a decrease in temperature. The cell was thermostated with silicon oil, whose temperature (+ 05°C) and circulation were controlled by a Hubert T 200 thermostat.

RESULTS

AND

DISCUSSION

The electrode reactions of the investigated cell Li/l mol kg ’ LiCIOL in DMS02/Li,V,0S are: cathode: anode:

V205

fxe-

xLi *xLi+

+xLi+

=Li,V,Os;

+ xe-,

where x = degree of insertion of lithium, overall basic reaction of the cell is: V,O,

+ xLi $

and

the

Li,V,O,.

In a previous work, it has been proved that the . ._ Vz0,/Li,Vz05 redox couple behaves as a reversible and Nernstian system[25]. Hence, thermodynamic information on insertion compounds Li,V205 were obtained from e.m.f.-temperature measurements of cell (A) on the basis of the following equations:

1 ASLi,x,T) = F (aE/‘aT)x 3 FC~Wa% -El, AH Li(x.T) = AC Li(x.rr = - FE<,,,,

(1) (2) (3)

where Ae L,,x,r AH L,(x) and AsLiu, are the partial molar Gibbs free energy, enthalpy and entropy of insertion of Li in the V,Os host lattice, respectively. E.m.f.-temperature measurements were performed in the temperature range 12(t17O”C. Some typical e.m.f.-T curves are shown in Fig. 1 for different x values in Li,V,O,. Linear variations are observed throughout the whole x-range investigated and in almost all cases the e.m.f. has a positive temperature coefficient. The values of the partial molar entropy of insertion of Li, AsLl(.+ obtained from these measurements according to Equation (2) are shown in Fig. 2. For very low lithium contents x d 0.025, high values of entropy, are found, which reflects a high degree of disorder ie a high mobility or a high vibrational freedom of Li in the insertion compound, as in the case of silver and lithium in the j?-V,05 host lattice[2&22]. Thus, lithium can be considered as a solute species dissolved in the V,05 solvent. With further reduction, filling the vacant sites with lithium results in a slow decrease of AsLi up to x = 0.15 and then in a sharp entropy change. It is obvious from the behaviour of this thermodynamic function that ordering increases in Li,V205 as x increases until a maximum which takes of AsLi is place when the minimum value reached, ie for x = 0.25/O-30. Beyond x = 0.3 a high degree of disorder appears until x = 0.85; thereafter ASri diminishes for higher lithium contents which suggests that a new ordering process occurs from x estimate AS = = 0.80/0.85. of - 25 J K-i mol-‘nfor x = 1 given by Dicken$26] agrees well with this phenomenon. From the combination of Equation (2) with the theoretical expression of the e.m.f.-x relationship for the investigated cell Li,V,O,/Li+/Li, it follows that AsLi,xj = ASFi + R In (1 - X)/X,

(4)

and AsFi is the partial molar where X = X/X-~ entropy of Li for the standard condition X = 0.5. Whatever the ordering process the high discrepancy observed between experimental and calculated values of AS,, according to Equation (4) proves that an additional entropy contribution should be added to Equation (4). It can be assumed that energy contributions arising from the repulsive coulombic interactions between lithium ions or from the lattice expansion should be taken into account and added to the concentration dependent term R In (1 - X)/X and then to the e.m.f.-x relationship. From the experimental As,, xj data and from the potential variation E us x (Fig. s ), using Equation (3), the partial molar enthalpy of insertion of Li in V,O,, Aflri is determined at 150°C as a function of x (Fig. 4). The contribution of the enthalpy AH ri to the activity of the lithium ions inserted into the V,Os framework is dependent on several factors. Indeed, the enthalpy of Li insertion includes the energy arising from lithium strong bonding to the oxide structure, an interaction energy which arises from the repulsive interactions between the inserted alkali ions and the energy required, to distort the crystal framework sufficiently to accommodate the inserted Li+ ions. 1AH,,1is seen to increase while As,i,xj is decreasing up to a value for which a maximum ordering of Li+ ions takes place in V,05. Qualitatively, this is in accordance with the filling of vacant sites leading to a

Electrochemical lithium insertion into vanadium pentoxide 1005

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Fig. 1. E.m.f.-temperature behaviour of the Li/l mol kg-’ = 12~170°C~.

I

1

0.1

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0.3

I

I

OS IX in Li_ V_p,I

I

T%

LiClO .+ m molten DMS02/Li,V,0,

I

0.7

I

1

0.9

Fig. 2. Partial molar entropy of lithium as a function of x in Li,V,O,.

I

cell (T

I X

J. P. PEREIRA-RAMOS

1006

I 36 O-

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I

I

I

et al. ,

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I

I

- -A6Li(k~.mO~-’ )

E(V)

3.70

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3.50

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3.30

3OC I’

3.10

28C I-

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,

1

0.1

0.3

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#

0.5 IX I" Lix"+J

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Fig. 3. E.m.f. t)s composition curve for electroformed Li,V,O,

x

at 150°C.

360,

320'

280

I 0.1

I

I a3

I I 0.5 (X in Lix Vz05)

I

L 0.7

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Fig. 4. Partial molar enthalpy of lithium as a function of x in Li,V,Os decreasing number of available sites, ie to a loss of mobility of LI + ions up to x = 0.25/0.30 where JAHLil is a maximum. This means that the strongest bonding of Li+ to the V205 lattice occurs for x = 0.25/0.30, IAHLi 1diminishes first strongly and thereafter slightly to reach values lower than those achieved for very weak lithium concentrations. This can be explained by

I

I

0.9

at 150°C.

an increase of the contribution of coulombic interactions to AflLi for high lithium contents. Unfortunately, only two calorimetrically determined AfiLi results for lithium insertion into orthorhombic V20, are presently available (Table 1). A satisfactory agreement is obtained between these two sets of data. As expected, lithium bonding energies

Electrochemical

lithium insertion into vanadium pentoxide

slightly decreases, b remains nearly constant, and c, ie the separation between the V20s layers, increases. For 0 < x -z 0.15, the x-dependence of the partial molar entropy and enthalpy is consistent with the existence of the a-phase. From x = 0.15 up to x = 0.25/0.30, a strong decrease in AS,, implying an ordering process, can be correlated to the existence of the two-phase region a + E described by Murphy[27]. Finally, the x range wherein the E phase is reported could correspond to the entropy plateau found for 0.30 < x c 0.85. In addition, from ESCA measurements it has been shown that a significant decrease in the atomic ratio V/O occurs with discharge of V205 up to x = 0.2010.25, followed by a plateau and thereafter by a new decrease for x = 1[32]. These data supply evidence for two successive distortion processes of the oxygen lattice, at least near the surface layer. It is interesting to notice that the two states of maximum ordering characterized by the two minimal values of ASLi found in the study (the first one at x = 0.25/0.30, the second probably being at x = 1 as suggested by the estimate given in[26]), can be correlated with these two main structural rearrangements of the V,O, matrix.

Table 1. Calorimetrically determined AfiLi results for lithium insertion into orthorhombic VzOs Compound

AI?Li* (kJ mol-‘)

Li0.~VK& J&.45 VK%

-334*9 -328k3

aLit

(kJ mol-‘) -317k2 -305+2

*From reference[26]. +Present work.

are found to he significantly lower for the low temperature materials than for the high temperatures phases a, p, /?’ and y.[26, 271. Similar e.m.f.-T measurements have been performed in a propylene carbonate-based electrolytes in the temperature range 10-6OC. All the partial thermodynamic quantities have led to the same behaviour (Fig. 5). This shows that the determination of such thermodynamic data is strongly related to structural features of the insertion compounds. Thermodynamics

and structural

properties

The low temperature bronzes Li,V,O, are known to be prepared either chemically by using mild reducing agents[27-291, or electrochemically[30--311. Unlike the high temperature phases, their structure is very close to that of the parent oxide and stability diagrams for these Li,V205 as a function of x have been published[27]. For x ranging from 0 to 1, the orthorhombic unit cell is conserved but two one-phase regions are observed (a and E) and only one two-phase region tl + E. The progressive lithium intercalation results in a gradual change of the unit cell parameters: a

lot I-

, AsL, (J.mol-!

1007

CONCLUSION The partial molar functions AGritX), AA,,,,, and Asr+) of the reaction V,O, + xLi = Li,V,Os have been obtained from e.m.f.-temperature measurements. The x-dependence of these quantities are discussed in terms of structural characterization reported for the Li,V*Os bronzes. Two ordering-processes have been shown to take place at x = 0.25/0.30 and x = 1.

K-’

aa

40

0

I

0.1

I

1

O-3

I

I

0.5 IX il LiX VrOr)

I

I

0.7

Fig. 5. Partial molar entropy of Lithium obtained from e.m.f.-Tmeasurements Li/l M LiClO, .in PC/Li,V,Os.

I

I

0.0

I

I

x

between 10 and 60°C in a cell

1008

.I. P. PEREIRA-RAMOSet

Further works are in DroPress in our laboratorv to establish a theoretical mod; of Li insertion[33] aid to investigate the detailed structural changes occurring in LixVz05. Acknowledgement-Financial support by the Direction des Recherches, Etudes et Techniques (DRET, decision no 86186) is gratefully acknowledged.

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al.

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