STRUCTURAL, ELECTROCHEMICAL AND THERMAL STUDIES OF DIVALENT POLYMER ELECTROLYTES

The main aims of this project were to produce a polymer electrolyte, containing a divalent cation, with a sufficiently high and stable ionic conductivity at room temperature to be used in a solid-state battery, and to gain a better understanding of the relationship between structure and conductivity. Of the 'dry' systems studied, the PEOn-ZnCl2 samples had the highest conductivities and transference numbers, although at room temperature, the conductivities were only about 10"^ S cm'’. Conversely, samples containing magnesium salts had lower conductivities, similar to pure PEO. Plasticised samples of PEO30:PCn:ZnX2 (PC=propylene carbonate; m=3.3 and 6.7; X=Cl,Br,I> were prepared. None of the films were tacky and, although they were quite flexible compared to films prepared without PC, all of them were mechanically strong. The addition of propylene carbonate dramatically improved the conductivity of all samples. The conductivity of the best sample, PEO30:PCe.:ZnCl2, was 1.1x10“® S cm at 25oC, compared with 7.4x10-'° S cm-1 for the non-plasticised sample. A gradual loss of conductivity was noticed with prolonged storage of films in the glove box at room temperature. DSC results showed that the glass transition temperature, in the samples it was detectable, had been significantly lowered by the addition of propylene carbonate. EXAFS studies of the Zn (K) edge of the 'dry' PE0n:ZnX2 (X=Cl,Br,I) samples revealed that zinc cations were in environments independent of the overall stoichiometry of the samples. They were co-ordinated to two halide anions each and, in addition, to approximately four oxygen atoms where X=C1,I and six oxygen atoms where X=Br. DSC results confirmed that zinc cations were in environments independent of overall stoichiometry, as the glass transition temperatures of the samples were found to be similar throughout. By comparing these values with those of 'quenched' samples, it was found that the stoichiometry of the amorphous phase of 'normal' samples was roughly 6:1 for X=CI,I and 8:1 for X-Br. Transference number measurements made on the 'dry' samples indicated that zinc species were mobile and magnesium species were not. In the PEOn:ZnCIz, samples, the zinc transference number was estimated to be between 0.15 and 0.45. Estimation of the electrode/electrolyte interfacial thickness in Zn/PEO-zinc halide/Zn cells yielded values lower than had been reported for PEO-lithium systems, even for the PEO30:PC3, ZnCl2 sample.

Name dc:type.qualificationname

PhD

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

De Montfort University

Year dc:date.issued

1989

Author dc:creator

• SHELDON, MARK HEDLEY

Licence

• https://dora.dmu.ac.uk/bitstreams/8e493480-c8b7-43d3-81a8-21d107a683d7/download