Ion-conductivity and Young's modulus of the polymer electrolyte PEO–ammonium perchlorate

doi:10.1016/j.ssi.2007.01.006

Solid State Ionics

Volume 178, Issues 3–4, February 2007, Pages 167-171

https://doi.org/10.1016/j.ssi.2007.01.006 Get rights and content

Abstract

We present a study of impedance spectroscopy (IS) under varying loads. An equivalent circuit is proposed taking the deformability and the effect of change in free volume into account, which yields the dc ion-conductivity as well as the Young's modulus from these results. Independent measurements of elastic properties are done and the result is compared with Young's modulus predicted from IS. The sample we studied is PEO–NH₄ClO₄ with a salt fraction of 0.19 by weight, with frequencies varying in the range of 40 Hz to 3 MHz and loads varying from 400 to 1600 g.

Introduction

Ion-conducting polymers may be used in solid state batteries as electrolytes [1], [2]. Hence there is an extensive literature on conductivity measurements [3]. The polymer electrolytes consist of a host polymer such as polyethylene oxide (PEO) with negligibly small ion-conductivity and a salt added to provide charge carriers. The dc conductivity is extracted from ac measurements as these materials become polarised under dc. Measurements with varying frequency, i.e. ‘impedance spectroscopy’ (IS), also provide more interesting information on the microstructure [4], [5]. However IS for the polymers has certain inherent problems, which are usually not discussed. The present communication deals with these problems. We show that from our attempts to tackle them, we can in fact, extract additional information on elastic properties from IS measurements. It is difficult to get reliable values of elastic properties such as Young's modulus from conventional techniques, as we shall show.

The experimental arrangement used for IS must provide a measurement cell with the sample (usually in the form of a film) sandwiched between two electrodes. The polymer electrolytes are too fragile to support vacuum-coated metal electrodes. A coating of silver paint, if used as electrode, may short circuit the sample, if it enters micro-cracks. So spring loading is used to press the electrode onto the sample. The PEO film, when cast from a solution has a rough surface, because of its multi-phasic character [5]. So the electrodes normally do not provide good contact for IS measurements. Using a stiffer spring improves the contact, but it also reduces the thickness of the soft sample. These facts are usually not taken into account nor are the conditions of measurement stated clearly, though considerable work has been done on such materials [6]. To improve the contact one procedure is to heat the sample up to its melting temperature, then allow it to cool to the required temperature of measurement. This will however change the original morphology obtained during solution-casting, so effect of morphology on ion-conductivity is lost. The importance of this effect has been clearly brought out in [7]. If a stiffer spring is used, the sample dimension changes and this must be considered when reporting results. Besides these effects, it is also possible that the microstructure and hence conduction mechanism itself changes as a result of applied pressure. Variation of ion-conductivity with pressure may be due to the reduction in free volume, the problem is discussed in Gray [8], and references therein. Thus because of its large deformability, the elastic properties of a polymer–salt film affect impedance measurement strongly.

As an alternative method to obtain elastic as well as electrical properties, we carry out impedance spectroscopy (IS) measurements on the polymer electrolyte PEO–NH₄ClO₄ under different loads and find a strong variation of admittance on loading in the range of 400–1600 g (3.92–15.68 N).

A brief description of the sample preparation and measurement techniques follows.

Section snippets

Sample preparation

Poly-(ethylene oxide) from B.D.H., England (mol wt. 6 × 10⁵), methanol (99.9% pure), ammonium perchlorate (Fluka, 99.5%) have been used for sample preparation. The polymer films have been prepared by the solution casting technique after dissolving the solid components in methanol. The suspension in methanol is stirred for 14–16 h at room temperature (23–25 °C). Films were cast on a Petri dish, dried in air for 4–5 days and then vacuum-dried for 3–4 days, under a pressure of 0.1 mbar. Vacuum

Analysis of the results

Results are analysed taking into account the fractal morphology, the large deformability of the soft samples and possible changes in internal structure due to loading.

Discussion

Inspection of the admittance versus weight curves shows a variation in shape for different frequency ranges. At the lowest frequency, the curvature is upward, while at 3 MHz, the average slope is downward throughout, though there are some fluctuations. In the intermediate frequency ranges, competing upward and downward trends lead to non-monotonic behaviour. The agreement of calculated curves with experimental results is quite satisfactory considering the simplicity of the equivalent circuit

Acknowledgements

MM and MS thank BRNS and UGC-DAE for providing research fellowships, through project Nos. 2002/37/8/BRNS and CRS-092/JU/P/ST/7394, respectively. Authors thank A. J. Bhattacharyya for helpful discussion.

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Ion-conductivity and Young's modulus of the polymer electrolyte PEO–ammonium perchlorate

Abstract

Introduction

Section snippets

Sample preparation

Analysis of the results

Discussion

Acknowledgements

Solid State Ionics

Prog. Solid State Chem.

Physica, A

Electrochim. Acta

Impedance Spectroscopy

Ionics

Phys. Rev., B