microstructure and electrical conductivity of complex cathodes based on new vanadium oxides and...

Die Angewundte Mukromolekulure Chemie 238 (1996) 31-39 (NK 4148)

Instituto de Polimeros, C.S.I.C., c/Juan de la Cierva 3,28006 Madrid, Spain

Microstructure and electrical conductivity of complex cathodes based on new vanadium

oxides and polyether blends

Enrique Morales*, Jose Luis Acosta

(Received 13 July 1995)

SUMMARY This paper studies the effect produced by different vanadium oxides on the micro-

structure of PEO and PEOPPO blends by means of the analysis of the crystallization kinetics under isothermal and nonisothennal conditions. The electrical proper- ties of the samples were characterized by complex impedance analysis. Results suggest that the additives employed (considering both carbon black and vanadium oxides) promote changes in the microstructure of PEOPPO polymer blends because of polymer-additive interactions. All samples tested show good conductivity proper- ties.

ZUSAMMENFASSUNG: In dieser Arbeit wurden durch die Untersuchung der Kristallisationskinetik unter

isothermen und nichtisothermen Bedingungen die Auswirkungen verschiedener Va- nadiumoxide auf die Feinstruktur von PEO und PEOPPO-Mischungen untersucht. Die elektrischen Eigenschaften der Probekorper wurden durch komplexe Impedanz- analyse ermittelt. Die Versuchsergebnisse bestatigen die Annahme, daB die verwen- deten Zusatze (RUB und Vanadiumoxide) aufgrund der Wechselwirkungen zwischen den Polymeren und den Additiven mikrostrukturelle Veranderungen im PEOPPO- Polymerverbund hervorrufen.

Introduction

The production, storage and distribution of energy are among the main concerns of modern industry and society. Vanadium oxides have been widely used in primary and secondary batteries due to their availability and ease for lithium intercalation, V205 and V60,, being the two most com- monly used'. Characteristics of V20, as a cathode component are slow dis- charge process2, poor reversibility (although in some systems complete re-

* Correspondence author.

0 1996, Hiithig & Wepf Verlag, Zug CCC 0003-3146/96/$07.00 31

E. Morales, J. L. Acosta

versibility has been ~bserved)~, low electrical conductivity4, and poor stabi- lity towards V205 of the organic solvent of the p~lyelectrolyte~. Addition- ally, structural defects have been observed because of lithium insertion, a fact that is undesired in electrochemical cells, since they distort the orthor- ombic cell unit6, breaking the V-0 bond, forming irreversible L i - 0 bonds, etc.

Cells based on V6013 present higher charge density, and an excellent sta- bility towards V601, of organic solvents as methyl formate, 1,2-dimethoxy- ethane, 4-butyrolactone, etc. compared to V202-9. Thus, one of our objec- tives has been the synthesis and characterization of stoichiometric and non- stoichiometric vanadium oxides located between V205 and V204 to increase the number of life cycles, the reversibility and the energy density of the cathodes", ". Blends of poly(ethy1ene oxide) and poly(propy1ene oxide) have been described to form a miscible blend, the system being phase-sepa- rated as molecular weights of both polymer components increase12. In this work the electrical conductivity and crystallization kinetics under isothermal and nonisothermal conditions of polymer cathodes based on poly(ethy- lene oxide) and its blends with poly(propy1ene oxide), carbon black (CB) and the different vanadium oxides V,O, are examined.

Experimental

Poly(ethy1ene oxide), PEO, was supplied by Aldrich-Chemie (M, = 5 * lo6, T, = 65°C). Poly(propy1ene oxide) was PAREL 58, obtained from Zeon Chemical Inc. (M, = 1.5 * lo6). Carbon black ISAF (20-25 pm) was supplied by Cabot. Vanadium oxide A (V20,) was a Merck product. Vanadium oxide B was obtained from V205 reduction in an oven at 450°C during 2 h under constant flow of N2-H2 gas (90/10, v/v). The vanadium oxides C and D were obtained from NH4VO3 reduction in a her- metically sealed oven under constant N2-H2 (90/10, v/v) gas flow for 1 and 2 h, re- spectively, at 450°C.

X-ray diffraction patterns of the different vanadium oxides were performed with a Siemens D-500 diffractometer using a Ni-filtered CuKa X-ray beam at 40 KV (Fig. l), showing marked differences in the crystalline structure.

Polymer cathodes containing 45 wt.-% of polymer (or polymer blend), 10 wt.-% of carbon black and 45 wt.-% of vanadium oxide were prepared by dissolving the polymers in acetonitrile. Once the polymers had been dissolved, the vanadium oxide powder and the carbon black were added to the solution under vigorous stirring. Then the dispersion was introduced in an ultrasonic bath to destroy the aggregates, getting a uniform dispersion. Films were then obtained by casting on PTFE plates and dried under vacuum at 50°C during 48 h. Thermograms were recorded on a Mettler TA4000 differential scanning calorimeter operated under nitrogen. For cool-

.

32

Microstructure and electrical conductivity of complex cathodes

- c c -

0

Vanadium: .- t

L

D I I I I I I I I

c !! B 4 -.--ALL- .- e z 1000 ).

ul t m

c I - .-

ing experiments, samples were heated to 100°C and held there for 10 min to erase any thermal history, cooled down to -20 "C at several cooling rates (1, 5 and 10 "C/ min), then heated up to 100°C at a heating rate of lO"C/min. Samples for isothermal crystallization were reheated at 100°C and held there for 10 min, then cooled to the desired crystallization temperature T,. Once the crystallization had been finished, melting temperatures were obtained by heating the crystallized samples at 10°C/ min. The analysis of the thermograms with two or more overlapping peaks was made using a deconvolution software adjusting a five parameter equation.

Complex plane impedance and inductance analysis were conducted at room temperature in an impedance analyzer (Hewlett-Packard model 4192 A) coupled to a computer, in the frequency range of lo6 to 1 Hz.

Results and discussion

Sample compositions are compiled in Tab. 1. Fig. 2 shows the melting thermograms of cathodes containing PEO and the four vanadium oxides tested, crystallized from the melt at -5 "C/min, and Fig. 3 shows the crystallization thermograms from the melt at -10 "C/min of PEO/PPO 50/50 blends containing the four vanadium oxides. Some thermograms show two crystallization peaks, thus suggesting that different structures are present in the crystallized cathode because of the interaction of the polymer with the vanadium oxide. It appears that the PEO morphology in cathodes containing V,O, is similar to that of the pure polymer, while the other vanadium oxides tested show displacements on the melting temperature. The blends behave in a similar manner.

33

E. Morales, J. L. Acosta - CAT - 1Al

CAT-181

CAT-1CI

I I I 25 50 75 li

T (OC)

Fig. 2.

CAT-501

0 50 1 T ("C)

Fig. 3.

0

Fig. 2. Melting thermograms of cathodes containing PEO and the four vanadium oxides tested, crystallized from the melt at -5 "C/min.

Fig. 3. Crystallization thermograms from the melt at -lO"C/min of cathodes containing PEO/PPO 50/50 blends and the four vanadium oxides tested.

The crystallization kinetics of polymers can be followed by using the Av- rami e q ~ a t i o n ' ~

x, = 1 - exp(-Kt") (1)

where x, represents the relative volume crystallinity, determined experimen- tally by integration of the crystalline conversion vs. time plot, K is a rate constant related to the kinetics of nucleation and growth, and the exponent n depends on the type of nucleation and on the geometry of growing crystals. The values of n and K are obtained from the graphic plot of log {-ln(l-xc)} versus log t.

Tab. 1 compiles kinetic data and melting parameters obtained for cathodes crystallized from the melt at S"C/min. Results indicate that the log K values of cathodes containing V205 are higher than those of the pure polymer because of nucleating effect of the vanadium oxide. The differences found for the n values are probably due to heterogeneous nucleation effects.

34

Tab.

1.

Kin

etic

par

amet

ers a

nd m

eltin

g da

ta fr

om c

atho

des c

ryst

alliz

ed fr

om th

e m

elt a

t -5

"C/m

in.

%

Sam

ple

Com

posi

tion

Non

isot

herm

al ki

netic

dat

a M

eltin

g be

havi

our

E. a 2

(PEO

/PPO

)N,O

,/CB

Pe

ak 1

Pe

ak 2

Pe

ak 1

Pe

ak 2

z

wt.-

%

5 ("

C)

(Jk)

("

C)

(Jk)

&

PE05

10

0(50

/50)

/0/0

2.

97

0.51

- -

67.6

12

6.6

-

-

B CA

T-1B

1 45

(100

/0)/4

5/10

4.

76

0.43

4.

66

1.22

56

.5

141.

0 65

.8

2.3

& ;E: a

n -lo

g (W

min

-I)

n -lo

g (W

min

-I)

T,

AH

Tm

AH

rn

-

2

I? PE

08

100(

80/2

0)/0

/0

3.09

0.

01

- -

68.2

13

1.0

-

-

2.

CAT-

1A1

45(1

00/0

)/45/

10

3.60

0.

03

PEO

1 O

O( 1

00/0

)/0/0

3.

20

0.38

- -

68.7

13

2.9

-

0 0

- -

68.6

10

7.1

-

-

ff -

s.

u" 2.

-

CAT-

1C1

45(1

00/0

)/45/

10

3.93

1.

69

- -

50.7

98

.4

-

CAT-

1D1

45(1

00/0

)/45/

10

4.31

1.

38

2.70

0.

01

55.4

94

.7

-

CAT-

4A 1

45(8

0/20

)/45/

10

3.00

-0

.32

- -

68.4

14

4.2

-

-

CAT-

4B 1

45(8

0/20

)/45/

10

3.32

0.

42

1.90

-0

.29

54.5

15

1.4

-

-

% 3.

79

0.04

-

-

56.3

14

3.0

-

-

5 z CA

T-4D

1 45

(80/

20)/4

5/10

2.

99

-0.1

4 1.

76

-0.2

4 51

.7

107.

5 70

.6

1.7

E 5 5

45.5

90

.6

65.7

4.

1 R

CAT-

4C 1

45(8

0/20

)/45/

10

CAT-

SA 1

45(5

0/50

)/45/

10

4.04

-0

.31

- -

66.1

11

3.8

-

-

CAT-

5B 1

45 (5

0/50

)/45/

10

3.84

0.

23

2.36

-0

.41

59.7

17

4.2

-

-

C A

T-SC

1 45

(50/

50)/4

5/ 10

3.

16

0.53

- -

CAT-

SD1

45(5

0/50

)/45/

10

2.67

-0

.12

2.39

-0

.23

51.1

88

.9

70.7

4.

0

0

lo


- c u)

C 3

* .-

& PEO L c .-

c .- g CAT-461 c -

- I * L.

0 Y-AJL-- c L

c .- g CAT-461 c -

I I I

2 B/degrees

Fig. 4. X-ray diffractograms of cathodes containing vanadium oxide B

10 20 30 40

It has to be noted that the nucleating effect decreases because of thermal treatment. Cathodes containing vanadium oxides B and D show two peaks on the crystallization thermogram, indicating the existence of two different structures in the crystallized sample; melting temperatures suggest the existence of a small amount of “free” PEO, not influenced by vanadium oxide interaction, and a new crystalline structure resulting of polymer-vanadium oxide interactions (Fig. 4).

Fig. 5 shows the plot of log K vs. cooling rate for PEORPO 80/20 cathodes; log K values for CAT-4B 1 and CAT-4D1 are those corresponding to the main peak (the one associated with a higher area). Cathodes having other polymer compositions present the same tendency. The results show that the differences observed between the different cathodes decrease with increasing cooling rates.

Tab. 2 and 3 show the kinetic parameters obtained under isothermal crystallization, as well as the melting data, for the cathodes containing vanadium oxides A or B. It has to be pointed out that crystallization temperatures dif- fer from one sample to another, which makes it impossible to compare quantitatively kinetic effects; however, a qualitative analysis has been made. Looking at the data in Tab. 2, no differences related to the n values were observed, that is, the type of nucleation and the geometry of the growing crystals seem to be independent of the PPO concentration in the blend for cathodes containing vanadium oxide A, while K decreases continuously as the PPO concentration in the blend increases. Cathodes containing vana-

36


I I I I I I I 1 0 4 8 12

Cooling rate (OC/min)

Fig. 5. Log K vs. cooling rate for PEOPPO 80/20 blends containing the four vanadium oxides tested; (0) PEOPPO 80/20, (+) CAT-4A1, (m) CAT-4B1, (0)

CAT-4C 1, (0) CAT-4D 1.

dium oxide B present a different behaviour (Tab. 3) (two peaks were observed when melting the crystallized sample). Isothermal crystallization of those samples show only one peak, because the secondary peak crystallization is finished at this crystallization temperature.

Tab. 2. Melting behaviour and kinetic data for isothermically crystallized cathodes containing vanadium oxide A.

Sample T, Isothermal kinetic data Melting behaviour ( "C)

n -log(Wmin-') tl,: T,("C) AH (min) (Jk)

CAT-lA1 53 54 55

CAT-4A1 53 54 55

CAT-5Al 51 52 53

2.38 2.08 2.43 2.72 2.47 3.36 2.53 2.80 2.45 3.36 2.51 4.28 2.10 2.50 2.10 2.96 2.09 3.44

6.44 11.19 19.78 10.97 20.16 43.75 13.03 21.49 37.68

70.0 70.1 70.5 69.6 69.9 70.3 67.6 67.7 68.3

94.4 85.1 76.4

115.5 113.3 113.3 111.5 104.4 113.3

a Half-time of crystallization.

37


Tab. 3. Melting behaviour and kinetic data for isothermically crystallized cathodes containing vanadium oxide B.

Sample T, Isothermal kinetic data Melting behaviour ( "C)

Peak Peak 1 Peak 2

CAT-1BI 45 1.94 1.85 46 1.96 2.33 47 1.92 2.83

46 1.90 2.44 47 1.82 2.80

44 2.13 1.97 45 1.99 2.59

CAT-4B1 45 1.97 1.97

CAT-5B1 43 2.04 1.67

7.4 57.7 12.9 57.9 25.5 58.6 9.4 56.0

16.0 57.0 29.5 58.0 5.6 58.7 7.3 59.3

16.8 60.4

120.5 65.3 2.2 98.9 64.8 3.2 90.0 65.0 3.8 8.0 63.3 108.4 7.9 63.7 106.2 7.2 64.4 92.8

96.7 66.9 6.8 96.2 67.0 6.8 98.3 67.9 7.9

Fig. 6 shows electrical conductivity at room temperature of the cathodes against polymer composition. Electrical conductivity decreases as PPO concentration in the blend increases for cathodes containing vanadium oxides A or D, while an increase in conductivity with increasing PPO concentrations was observed for cathodes containing vanadium oxide C . Cathodes contain-

1 2.OE-2

40 60 80 100 PEO (wt.-%)

Fig. 6. Electrical conductivity of the PEOPPO samples at room temperature; (0 )

VA, (0 ) VB, (0) VC, (+) VD.

38


ing vanadium oxide B present a different behaviour, the value of cr reaches a minimum for PEOPPO 80/20 blend. The highest value was obtained with samples containing vanadium oxide C with a polymer composition PEO/ PPO 50/50.

Conclusions

The microstructures of polymer cathodes based on poly(ethy1ene oxide) blends and different vanadium oxides have been studied through the analysis of kinetic parameters and melting behaviour under isothermal and nonisothermal conditions. Results suggest that morphological changes take place due to interactions between the polymer and the different oxides, resulting in changes in the nucleation morphology and growth of PEO spheru- lites. Electrical characterization results show that the conductivity depends not only on the nature of the vanadium oxide, but also on the nature and composition of the polymer system.

This work was supported by the Communidad Aut6noma de Madrid (Spain) under the Project C076/91.

K. M. Abraham, J. Power Sources 7 (198 1) 1 G. Voorn, J. Power Sources 14 (1985) 135 J. P. Pereira Ramos, R. Messina, J. Perichon, J. Power Sources 16 (1985) 193 K. West, B. Zachan-Christiansen, M. J. L. Ostergard, T. Jacobsen, J. Power Sources 20 ( 1987) 165 D. W. Murphy, P. A. Christian, Science 205 (1979) 651 S. Hub, A. Tranchant, R. Messina, Electrochim. Acta 33 (1988) 997 K. M. Abraham, J. L. Golman, M. D. Dempsey, J. Electrochem. SOC. 128 (1981) 2493 M. Uchiyema, S. Slane, E. Plichta, M. Salomon, J. Power Sources 20 (1987) 279 D. W. Murphy, P. A. Christian, F. J. Disalvo, J. N. Carides, J. Electrochem. SOC. 126 (1979) 497

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microstructure and electrical conductivity of complex cathodes based on new vanadium oxides and...

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