SOLID

ELSEWIER Solid State Ionics 95 (1997) 269-274

Microstructure and electrical properties of cathodes based on V,O,

Enrique Morales*, Jose Luis Acosta

STATE IOHKS

Institute de Ciencia y Tecnologia de Polimeros C.S.I.C. ClJuan de la Cierva 3, 28006 Madrid, Spain

Received 23 April 1996; accepted 12 November 1996

Abstract

Structural and electrical studies have been undertaken on cathodes composed of V,O,, carbon black and several solid

polyelectrolytes as binders, with the objective of examining the effects produced by lithium salt incorporation on the

morphology and conductivity of the complex cathodes. In addition, lithium insertion characteristics were studied and compared with that of the cathode in the absence of lithium salt.

Keywords: Vanadium oxide; Lithium intercalation; Batteries; Polyethers

1. Introduction

Electrochemical lithium intercalation into vana-

dium compounds has been widely studied due to

their application as cathode active materials for high

energy density rechargeable batteries and electro-

chemical devices. In this way, the performance of

cells with various cathode active materials including

Fe,V,O, [l], V,O,, [2-41, LiV,O, [4,5] and V,O, xerogel [4] has been described. Since electrochemi- cal intercalation and deintercalation are in general limited by the rate of diffusion of the lithium ion in

the oxide electrode, the attention of previous works

[6,7] has been focused on the determination of diffusivity in the electrode material. In contrast the

mechanism by which lithium ions cross the elec- trolyte-electrode interface has receive little attention. In this paper, we study the microstructure, electrical conductivity and lithium insertion of complex

*Corresponding author.

cathodes containing orthorhombic V,O,, carbon

black and a solid polymer electrolyte compl- exed with lithium trifluoromethanesulphonate

(LiCF,SO,). The purpose of the study was to check

if the movement of the lithium ions through the

interphase is favoured by using compatible polymer

systems in the cathode and the solid polyelectrolyte,

that is the use of the solid polyelectrolyte as binder. Complexes formed with poly(ethylene oxide) (PEO)

and various alkali metal salts are well known as ionic conductors [8,9]. The conductivity of PEO- LiCF,SO, complexes was reported to be in the range

10m9 to lo-’ at 100°C. Due to the fact that lithium

ion mobility only takes place in the amorphous regions, efforts have been made to increase ionic

conductivity, reducing crystalline content by blend- ing with other polymers or incorporating plastizicing additives to the complexes [lo-121. In this way, we

study solid polyelectrolytes composed of blends of poly(ethylene oxide), poly(propylene oxide) and a fluorinated polyphosphacene.

0167-2738/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved

PII SO167-2738(96)00596-6

270 E. Morales, J.L. Acosra I Solid State Ionics 95 (1997) 269-274

2. Experimental

Poly(ethylene oxide), PEO was supplied by Al- drich-Chemie (M, = 5.106, T,,, = 65°C). Poly-

(propylene oxide) was PAREL 58, obtained from

Zeon Chemical (M, = 1.5 - 106). Poly(octofluoropen-

toxytrifluoroethoxyphosphazene) (PPz) was supplied

by Firestone (PNF200). Carbon black ISAF 200

(20-25-pm average diameter) was supplied by Cabot. Vanadium oxide V,O, was a Merck product.

Propylene carbonate (PC) supplied by Aldrich was

stored under a 4 A molecular sieve. Lithium tri-

fluoromethanesulfonate (LiCF,SO,) was an Aldrich

product, used as received. Polymer electrolytes were prepared by dissolving

a known weight of LiCF,SO, in acetonitrile, then a

known weight of PEO was added with constant stirring for 24 h. Films were prepared by casting

over PTFE plates, then vacuum dried at 50°C for 48

h. The ratios O/Li+ employed were 8:l and 4:l. Composite cathode slurries, containing 45 wt% of

polymer (or polymer blend), 10 wt% of carbon

black, 45 wt% of vanadium oxide and the corre- sponding amount of lithium triflate were prepared by

dissolving the polymers and lithium salt in acetoni-

trile with constant stirring for 24 h. Films were then obtained by casting on PTFE plates and dried under

vacuum at 50°C during 48 h. All manipulations were

made inside an argon-filled glove box, with a

humidity level lower than 0.4 ppm.

Pellets for conductivity measurements (6 mm

diameter, 0.2 mm thickness) were compression molded at 100°C in a cell similar to that used for Ir

pellet preparation, provided with vacuum and a heating element.

Thermograms were recorded on a Mettler TA4000

differential scanning calorimeter operated under ni-

trogen. Samples were loaded in hermetically sealed aluminum pans, held at 220°C for 5 min to erase any

thermal history, and then annealed to - 100°C. The melting behavior of the samples was determined by heating up to 220°C at a heating rate of lO”C/min.

Complex plane impedance and inductance analysis were conducted in an impedance analyzer (Hewlett- Packard model 4192 A) coupled to a computer, in the frequency range of lo6 to 1 Hz, applying a heating program geared through an Oxford Instru- ments ITC 4 temperature controller. The sample is

placed in an Oxford Instruments DN 1710 cryostat

provided with a liquid nitrogen chamber, specially

designed for measurements at variable temperature. The electrochemical characterization was per-

formed in a cell with a lithium-aluminum alloy (5

wt% of aluminum) as the negative electrode, a

plasticized solid polymer electrolyte (PEO/PPz 80/

20, O/Li+ 8: 1, 80% phr of propylene carbonate), and the composite cathode containing the solid elec-

trolyte (PEO/PPz 80/20), with an O/Li+ 8:l (Cell

1). The function of the plasticizer was to increase the ionic conductivity of the electrolyte and subsequently

a lower temperature of operation. The loading of

active material was 37 g/cm’. The electrode area

was 1.13 cm’, and the whole assembly was placed in

a sealed PTFE holder. Stainless steel was the materi-

al used for blocking electrodes. The cell was then placed in an oven at a controlled temperature (60°C).

Lithium insertion was then monitored using a poten-

tiostate-galvanostate MacPile 11 coupled with a computer for data analysis.

For comparison another cell was tested; Compo-

site cathode contains V,O, (65 wt%), carbon black (10 wt%) and a PEO/PPz 80/20 polymer blend (15

wt%) as a binder. The polyelectrolyte was PEO/PPz

80/20 with an O/Li+ ratio 8:1, again plasticized with a 80 wt% of propylene carbonate. Lithium

insertion was then studied at 60°C.

3. Results and discussion

3.1. Thermal behavior

The DSC thermograms of PEO and solid polymer electrolytes based on PEO and PPO blends solvated with LiCF,SO, systems are shown in Fig. 1. The

complexes show two melting transitions, one about 65°C and the other one located between 154 and

189°C. This behavior suggests that two crystalline phases coexist at room temperature; a pure polymer phase, which melts at = 65°C and a crystalline

polymer-salt complex phase, being the extent of

each phase, that is, the peak enthalpy dependent of the value of x. Blends have the same behavior. Table 1 compiles the values of glass transition temperature, measured at the midpoint of the C, jump and melting parameters of the different samples.

E. Morales, J.L. Acosta I Solid State Ionics 95 (1997) 269-274 271

0 200

Fig. I. DSC thermograms of the PEO and PEO/PPO 80/20 based

cathodes.

The glass transition of the complexes are shifted toward higher temperatures, as expected due to the

lower degree of chain movements due to oxygen

Table 1

Thermal parameters obtained for solid polyelectrolytes

coordination, except for polyelectrolytes containing

PPz, being the Tg values of these samples indepen- dent of lithium salt concentration, thus indicating

that there is no coordination between the ether

oxygen of the lateral chain with the lithium cation. For samples E4-4 and electrolytes based on PEO/

PPz blends, no glass transition was detected, due to

the small jump taking place on Cp. It must be

pointed out that when x = 4, that is at the higher

lithium salt concentration studied, the melting peak of pure PEO disappears, or has a low enthalpy value,

thus suggesting that crystalline phase is mainly formed by complexed polymer. When x = 8, the pure

PEO enthalpy increases, being the melting tempera- ture of the complex shifted toward lower tempera-

tures.

Analogous previous studies on the microstructure

of cathode precursors [ 13- 151 based on V,O,, carbon

black and PEO, PPO, PPz and its blends as poly- mers, show only one melting peak on the thermo-

grams, located close to the melting temperature of pure PEO for the complex based on PEO, while for

the blends a decrease on the melting temperature was observed, this magnitude being a function of poly-

Sample

El-0

El-8

El-4 E2-0

E2-8

E2-4

E3-0

E3-8

E3-4

E4-0

E4-8

E4-4 E5-8

E5-4 E6-0

E6-8

E6-4 E7-0

E7-8

E7-4

Composition

PEOIPPOIPPz

100/o/o

100/o/o

100/o/o o/100/0

o/100/0

0/100/0

o/o/100

o/o/ 100

o/o/ 100

80/20/O

80/20/O

80/20/O 50/50/o

50/50/o 80/O/20

80/O/20

80/O/20 50/o/50

50/o/50

50/o/50

O/Li+

0

8

4 0

8

4

0

8

4

0

8

4

8

4 0

8

4 0

8

4

Thermal behavior

T, T In.1 Wn,, Ah W) (“Cl (J/g) (J/g) - 51.7 68.4 130.2 _ _

- 34.7 68.3 63.2 154.0 35.9

24.1 57.1 11.7 183.8 73.5 - 60.5 _ - 14.6 _ _ _

22.6 _ _ _ -61.1 _ _

- 60.5 _ _ _ -60.1 _ _

65.5, - 53.2 68.2 138.6 _ _

- 64.4 61.3 21.2 163.4 42.0 _ _ 185.3 13.0

- 60.6 67.5 1.9 178.8 42.3

13.0 189.1 43.0 - 66.4, - 49.2 67.6 152.8 _ _

_ 67.3 28.3 159.5 30.9 _ 188.8 63.4

- 60.9 67.0 139.9 _ _

_ 68.3 24.6 156.6 24.4 _ 185.3 52.9

272 E. Morales, J.L. Acosta / Solid State Ionics 95 (1997) 269-274

mer blend composition. The glass transition tempera- ture of the samples was displaced towards higher temperatures relating to that of the pure polymers, a result of the restrictions of polymer chain movement due to the solid V,O, particles.

Table 2 presents thermal parameters obtained for lithiated complex cathodes. Comparing data with those obtained for solid polyelectrolytes it is noted than T, values are higher than those of the corre- sponding polyelectrolytes, that is V,O, contributes to restraint in the movement of the molecules in the amorphous phase in accordance with data obtained for cathode precursors. On the opposite site, melting temperatures for complex cathodes are lower than those observed for analogous polyelectrolytes; this is explained as a result of the formation of imperfect crystals of lower size during the crystallization process.

3.2. Electrical characterization

Previous studies [ 14,151 on the electrical charac- terization of cathodes based on PEO, PPO, PPz, V,O, and carbon black concluded that the conductivity depends on the composition of the polymeric system. The conductivity of all the cathodes, whose com- positions are listed in Table 2, was determined by means of complex plane impedance spectroscopy, using nickel as blocking electrodes. All samples

yielded an arc, from which their conductivity was determined from the intercept with the 2’ axis. The values obtained are the sum of the contributions of the electronic conductivity due to the carbon black plus the ionic conductivity. The conductivity of all samples was observed to be temperature dependent. Fig. 2 shows the graphic representation of total conductivity (electronic plus ionic) versus l/T for cathodes containing polyelectrolytes based on PEO, PPO and PPz at the two lithium salt concentrations employed. All cathodes show an increase in con- ductivity value as temperature rises. Cathodes based on PEO with O/Li+ ratio x = 4 have conductivity values much higher than that with x = 8; cathodes

l/TxlOOO (K)

Fig. 2. Conductivity versus temperature for cathodes based on

PEO, PPO and PPz.

Table 2

Glass transition temperatures and melting parameters of the different cathodes tested

Sample Composition

PEOIPPOIPPz

T, T In.1 Wn,, T ml.2 Wn.z

w, CB OILi’ (“C) (“C) (J/g) (“C) (J/g)

lAl-8 1001010

lA1-4 100/o/o

2Al-8 o/100/0

2Al-4 0110010

3Al-8 o/o/ 100

3Al-4 0/0/100

4Al-8 80/20/O

4Al-4 80/20/O

5Al-8 50/50/o

5Al-4 50/50/o

6Al-8 80/O/20

6Al-4 80/O/20

7Al-8 50/o/50 7Al-4 50/o/50

45 10 8 - 40.2

45 10 4 _

45 10 8 - 24.6

45 10 4 22.1

45 10 8 - 62.2 45 10 4 - 63.7 45 10 8 - 45.8

45 10 4 36.1 45 10 8 - 31.2

45 10 4 29.4 45 10 8 - 46.6

45 10 4 _

45 10 8 - 52.2 45 10 4 - 64.4

44.4

51.5 _

_

_ _ 52.8 12.1

66.2 3.0

36.8 4.5 _ _

51.3 51.3

48.8 4.6

51.2 9.6

15.4 134.0 15.1

5.7 176.1 60.6

_ _ 151.6 17.3

173.6 37.3

135.4 15.6

161.9 25.2

139.6 12.7

173.5 42.1

143.5 9.8

172.3 30.9

E. Morales. J.L. Acosta I Solid State tonics 95 (1997) 269-274 273

based on PPO practically show no dependence

between conductivity and lithium salt concentration, in the range of lithium salt concentration tested,

while lines corresponding to log versus l/T repre-

sentation crosses for cathodes based on PPz. This behavior is explained in terms of changes

produced in the morphology. In this line PPO and

PPz are amorphous materials, and the changes

exerted by V,O, on its morphology are expected to

be of smaller extent than for cathodes based on PEO, where a new crystalline structure was formed as a

result of the lithium salt. Change of the O/Li+ ratio

leads to a new material with a different Tg, different

microstructure and a different log versus temperature

behavior and so with a different conductivity be-

havior. Figs. 3 and 4 show conductivity versus l/T plots

for cathodes based on polymer blends. Cathodes

-l-

--2- i- f - 2 -3-

2 - g-4-

s - J-5-

-l-

-2-

-3-

-4-

-5-

-4-T-T 4-r-T I/TxlOC!O (K)

Fig. 3. Conductivity versus temperature for cathodes based on

PEO/PPO blends.

2 3 2 V~xlOOO (Kf

3 4

containing PEO/PPO 50/50 show higher conduc-

tivity, probably as a result of the decrease on the

crystallinity level of the system than those containing

PEO/PPO 80120, the conductivity of cathode being based on PEO/PPO 50/50 with O/Li+ ratio x = 4

higher than that based on pure PEO (see Fig. 2). For

cathodes containing PEO/PPz blends, lines are again

crossed, indicating that the lithium salt concentration which leads to a higher conductivity level is tem- perature dependent.

It has to be noted however that values obtained

from electrical conduction for lithiated complex cathodes are slightly lower than those obtained for

cathode precursors, a possible explanation for this is

that the increase in the crystallinity content of the

complex cathodes results in solvated structures.

3.3. Lithium insertion

V,O, basically has a layer structure composed of the V-O-V sequence with weak V-O bonds across

the layer [16]. When V,O, is discharged, several steps are evident, each one associated with the

formation of a new phase as a result of lithium

incorporation. Fig. 5 shows the lithium ion insertion

diagram for cells 1 and 2. Both cells have a similar

open circuit voltage value (close to 3.5 V), and

insertion takes place between these values and 1 V

with an intensity value of 0.050 mA. First of all, it is clear that lithium insertion is much favored when

lithium salt is incorporated into the composite

cathode, a consequence of a decrease in the resist-

ance at the interface and to a greater degree in lithium permeation through the cathode phase.

3.9

;: > Cell 1

; 2.5 Z

= “O; b

r 2.0 ‘0 : 1.5 Call 2 0 0 1.0

0.2 0.4 0.6

XhYOS Fig. 4. Conductivity versus temperature for cathodes based on

PEO/PPz blends. Fig. 5. Lithium insertion diagram of cells I and 2

274 E. Morales. J.L. Acosta I Solid State Ionics 9.5 (1997) 269-274

Both cells show the same number of plateaus,

however the voltage at which these plateaus take place is substantially larger for the cell incorporating

lithium in the cathode. Further studies to analyze the mechanism and reversibility of each one of the

insertion stages are currently been done in our

laboratory.

Acknowledgments

This work was supported by the Plan National de

Investigation Cientifica y Desarrollo Tecnologico,

under the project Mat95 0203

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