Copper polymer electrolytes

SOLID STATE

Solid State Ionics 51 (1992) 215-218 North-Holland

IOHICS

Copper polymer electrolytes F. B o n i n o , S. P a n e r o , L. B a r d a n z e l l u a n d B. S c r o s a t i Dipartimento di Chimica, Universita di Roma "La Sapienza % Rome, Italy

Received 21 October 1991; accepted for publication 25 October 1991

Multivalent salt polymer complexes offer important prospects of the investigation and understanding of the fundamental properties of polymer electrolytes. In this work we present some recent results obtained by complex impedance, cyclic voltammetry and electron spin resonance on a series of polymer electrolyte systems based on the combination of poly (ethyleneoxide) PEO and copper salts of the C u ( C F 3 S O 3 ) 2 type. The data appear to confirm that copper ions contribute to the overall transport in these complexes. However, the mechanism of conductivity may also include a mixed ionic-electroniceffect.

1. Introduction Preliminary results obtained in previous works [ 14] suggested that the transport properties o f ( P E O ) x Cu(CF3SO3)2 p o l y m e r complexes are quite intriguing. Although copper ion transport has been shown by the Cu plating and stripping process on inert substrates [2 ], the exact m e c h a n i s m o f ion m o b i l i t y is still unclear. W i t h the aim o f obtaining further inf o r m a t i o n o n the t r a n s p o r t characteristics o f these copper p o l y m e r electrolytes, we have carried out a study, mainly based on cyclic v o l t a m m e t r y and frequency response analysis, on a series o f ( P E O ) x Cu (CF3SO3)2 complexes. D a t a o f electron spin resonance are also quoted in the discussion; these data have been o b t a i n e d within a collaboration with Prof. S. G r e e n b a u m o f the D e p a r t m e n t o f Physics o f the H u n t e r College, C U N Y , New York.

2. Experimental The ( P E O ) x - C u ( C F 3 S O 3 ) 2 complexes were prepared in the form o f thin films with a casting procedure from solutions o f a p p r o p r i a t e ratios o f copper salt (Fluka extrapure p r o d u c t ) and PEO ( 4 X 10 6 M.W., B D H p r o d u c t ) , following the m e t h o d described in details in previous works [ 1-4 ]. Both aceJ Author to whom all the correspondence should be sent.

tonitrile ( A N ) and methyl formate ( M F ) were utilized as solvents in the a t t e m p t to account for the possibility that traces o f A N m a y remain associated with copper ions in the casted films and thus introduce some uncertainties in the interpretation o f the transport results. The conductivity measurements were performed by sandwiching a few layers o f the selected p o l y m e r electrolyte sample films between two stainless-steel (SS) blocking electrodes. The conductivity was investigated between room t e m p e r a t u r e and 120 ° C by variable frequency (1 m H z - 6 5 kHz range) i m p e d ance analysis. The bulk resistance o f the films was d e t e r m i n e d by i m p e d a n c e analysis using a Solartron model 1250 frequency response analyser coupled with a Solartron model 1286 electrochemical interface, both connected with a HP-Vectra computer. The t e m p e r a t u r e control was assured by placing the cell in a Buchi m o d e l TO-50 oven. The current-voltage and cyclic voltammetry curves were o b t a i n e d by an A M E L model 551 potentiostat and an A M E L model 567 function generator. F o r the electron spin resonance procedure and the interpretation o f the related data, reference is m a d e to the specific papers [3,5].

3. Results and discussion ( P E O ) x - C u (CF3SO 3 ) 2 p o l y m e r electrolyte corn-

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F. Bonino et al. / Copper polymer electrolytes

plexes have been examined here in two compositions, namely x = 10 and x = 5 0 . Conductivity data for these complexes are displayed in form of Arrhenius plots in fig. 1. The data of fig. 1 refer to complexes prepared in MF; however, the results are essentially the same for samples prepared from AN. Melting of the crystalline phase at about 65°C is clearly manifested as a change of slope in the conductivity versus 1/ T curve for the x-- 50 sample. The higher conductivity of the x = 10 sample is attributable to its higher concentration of ions relative to the x = 50 sample. Also, the x = 10 sample apparently does not exhibit the amorphous to crystalline transition, possibly because this complex may retain most of its amorphous nature even at room temperature due to slow phase-requilibration kinetics. The Arrhenius plots furnish an overall measure of the conductivity of the polymer electrolytes; the determination of the reciprocal cationic, anionic and, possibly, electronic contribution requires additional investigation. Electron spin resonance (ESR) has been proved to be an useful tool in probing the extent of ion mobility in polymer electrolytes [5]. Therefore, this technique was also employed for the study of the conductivity of the (PEO)x-Cu (CF3SO3) 2 electro-

lytes here of interest [3 ]. Fig. 2 shows the effect of temperature on the intensity of the ESR signal for a x = 50 sample. Disregarding the change in the observed lineshape, the integrated intensity of the spectrum in fig. 2b taken at 67°C is about a factor of 3 smaller than that of fig. 2a taken at a low temperature ( - 9 7 ° C ) . Exchange effects could be a possible cause for the decrease in signal. Exchange effects resulting in line broadening can occur only when Cu 2+ ions approach each other to within 5 ~ or less [3]. Although it is tempting to attribute the occurrence of these close encounters to Cu 2+ ion mobility, it is also possible that they may be brought about by polymer segment-segment interactions in which the Cu 2+ ions remain connected to their respective segments. Therefore, and unfortunately, the ESR results are not conclusive in proving Cu 2+ ion mobility in (PEO)xCu (CF3SO3) 2 electrolytes. Further information in this respect may be obtained by frequency response analysis (FRA). Fig. 3 shows a typical ac impedance response at 65°C of the (PEO) ~o-Cu ( CF3SO3 ) 2 complex sample housed in a cell using two stainless-steel (SS) electrodes. If the transport would be purely by Cu ions, the SS electrodes would supposedly act as ionically blocking electrodes. However, the ac impedance response of fig. 3 does not evolve with the low frequency spike typically expected for blocking electrodes but rather expands on a semicircle. From the analysis of the

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-7 2.5

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\. 2.7

2.9

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Fig. 1. Electrical conductivity of (PEO)Io-Cu(CF3SO3)2 (squares) and of (PEO) 5o-Cu(CF3803)2 (stars) both prepared from MF.

Fig. 2. First derivative ESR spectrum of (PEO)so-Cu(CF3803)2 at -97°C (curve a) and at 67°C (curveb).

F. Bonino et al. / Copper polymer electrolytes

lO

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Fig. 3. Impedance plot at 75°C of a (PEO)~o-Cu(CF3SO3)2 electrolyte between two SS electrodes. Frequency range: 0.05 Hz25 kHz. (Crosses: experimental; circles: fitted. ) impedance parameters (using a non-linear leastsquares fitting program with a modified LevembergMarquardt algorithm adapted from the Boukamp [ 6 ] E Q I V C T original code), one can determine the values of the intercepts with the real axis, R b and Rot, as well as the top-semicircle capacitance, Cdt. However, while R b c a n be rather safely assigned to the bulk properties of the electrolyte and its value used for the determination of its conductivity (e.g. as in fig. 1 ), Rc~ and Cm are of more difficult explanation. The value o f Cdt (5.3X 10 - 6 F c m - 2 ) is typical of an electrode interface, the nature of which is not clear. One can only tentatively imagine a transfer process taking place at the electrode surfaces, such as a redox process between different copper oxidation states. Cyclic voltammetry (CV) of cells using SS working and Cu counter electrodes were carried out in the attempt of confirming this interpretation. Fig. 4 illustrates the voltammogram obtained in the voltage range of - 0 . 4 to 0.4 V: essentially one peak is here evidenced, probably related to the copper platingstripping process: Cu e+ + 2 e - ~ C u

1,4.

-0.3

( 1)

on the stainless-steel substrate. The voltammetric response, however, suggests that the reversibility of the process is poor. When the voltage scan is extended more cathodically, as for instance to - 0 . 6 V (fig. 4), a new peak appears, which may be tentatively attributed to the occurrence o f two competive (or consequential) electrochemical processes, namely:

-t

.5 ....... -1.0

i .......

-0.6

i .......

-0.2

Voltage

J .......

0,2

i .......

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(V)

Fig. 4. Cyclic voltammetry of the SS/(PEO)x-Cu(CF3S03)2/ Cu cell at 75°C and extended to different voltage ranges. Scan rate: 5 mV s-L SS working electrode, 0.25 cm2 surface.

Cu2+ + e - - ~ C u +

(2)

or Cu++e-~Cu,

(3)

where the lower oxidation state Cu ÷ cation can be promoted by the direct reaction of electrodeposited copper (process ( 1 ) ) with the Cu 2÷ copper ions from the electrolyte: Cu+Cu2+-~2Cu + .

(4)

In favour o f process (3) is the experimental evidence that when the voltammetric scan is extended into the cathodic region up to - 1 V (not shown in fig. 4), a rapid depletion in Faradaic current is observed, this being associated to the instability o f the lower oxidation state Cu + cation. In conclusion, the impedance and voltammetric results clearly suggest that the electrochemistry of the ( P E O ) x - C u (CF3SO3) 2 electrolytes is rather intriguing and, in particular, that the process at the electrode is more complicated than the simple deposition of copper from the polymeric media. In this respect, it is important to point out that the occurrence of process (4) promotes the coexistence of two redox states o f the copper ions, thus favouring electronic transport via electron hopping through the electrolyte. Indeed, evidence of a substantial electronic contribution to the conductivity o f the

218

F. Bonino et aL / Copper polymer electrolytes

( P E O ) ~ - C u ( C F 3 S O 3 ) 2 electrolytes, has been reported in ref. [ 1 ]. Considering the interest of all these aspects, which are certainly anomalous in respect to the general category of PEO-based polymer electrolytes, we are p l a n n i n g to perform further electrochemical and spectroscopical analysis with the aim of fully clarifying the nature and the related implication of the electronic transport in ( P E O ) x Cu (CF3SO3) 2 electrolytes.

Acknowledgement The ESR results here reported and discussed have been obtained within a collaboration with the Hunter College of C U N Y in New York. The authors are grateful to Prof. Steve G. G r e e n b a u m for the experimental data a n d the valuable discussion. This work

was been supported by the C o m m i s s i o n of the European Communities, u n d e r grant B R I T E - E U R A M Project, BREU-0167.

References [1] F. Bonino, S. Pantaloni, S. Passerini and B. Scrosati, J. Electrochem. Soc. 135 (1988) 1961. [2] S. Passerini, R. Curini and B. Scrosati, Appl. Phys. A 49 (1989) 425. [ 3 ] K.J. Adamic, S.G. Greenbaum, S. Panero, P. Prosperi and B. Scrosati, Proc. Solid State Ionics Syrup., Fall Meeting Mater. Res. Soc., Boston, USA, Nov. 26-Dec. 1, 1990. [4 ] F. Bonino,S. Panero, P. Prosperi and B. Scrosati, Electrochim. Acta, to be published. [ 5 ] K.J. Adamic, F.J. Owens, S.G. Greenbaum, M.C. Wintersgill and J.J. Fontanella, Proc. 2nd Intern. Syrup. Polymer Electrolytes, ed. B. Scrosati (Elsevier, London, 1990) p. 6 I. [6] B.A. Boukamp, Solid State Ionics 20 (1986) 31.