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Gelation and melting point depression in PVDF-based gel electrolytes

Salman, Raid Khider (2014) Gelation and melting point depression in PVDF-based gel electrolytes. PhD thesis, University of Leeds.

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Abstract

This thesis presents the designs and results of experimental investigations into gelation and melting point behaviour for polymer gel electrolytes. The study used one of the most prominent types of materials in the commercial world—polyvinylidene fluoride-based (PVDF) gel electrolytes—focussing on the effects of gelation temperature and gel composition on the properties of its solid electrolytes. While PVDF was used as the polymer matrix, propylene carbonates (PC) and diethyl carbonate (DEC) were used as solvents. Lithium tetrafluoroborate (LiBF4) and lithium bisoxalatoborate (LiBOB) were used as the electrolyte salts. Gelation from molten PVDF solutions was studied isothermally, using 30% PVDF/PC unsalted and 30% PVDF/PC/ LiBF4 salted gels to investigate the effect of salt addition on gelation behaviour. Crystallisation behaviour and subsequent melting was also investigated. Varying the gel composition involved changing polymer concentration, solvent nature and salt concentration. The measured properties included gel structure and morphology, melting point and ionic conductivity. The techniques of DMTA, DSC, WAXS, dielectric spectroscopy and optical microscopy were used to investigate gelation behaviour and gelation properties. It was found that gelation time is significantly increased at higher temperatures, and that the addition of salt reduces gelation time at any given temperature whilst extending the gelation process to higher temperatures. Gelation occurs without crystallisation at high temperatures, whereas it is induced by crystallisation at low temperatures. The sharp cut-off for crystallisation gives two distinct temperature windows, within which gelation occurs via different mechanisms. The addition of salt has found to raise the gel melting point by about 25C for 1M LiBF4, and hence to enhance its thermal stability, which is attributed to the salt affecting the Flory interaction parameter. Salt also reduces crystal size and gel pore size, and this is attributed to the nucleating effect of the salt molecules. The incorporation of up to 40% DEC in the solvent increases the gel melting point by 5 to 10 C, and is accompanied by a negligible reduction in ionic conductivity, which shows a possible route for further enhancing the properties of the electrolytes.

Item Type: Thesis (PhD)
ISBN: 978-0-85731-739-1
Academic Units: The University of Leeds > Faculty of Maths and Physical Sciences (Leeds) > School of Physics and Astronomy (Leeds)
Identification Number/EthosID: uk.bl.ethos.605552
Depositing User: Repository Administrator
Date Deposited: 07 Jul 2014 13:33
Last Modified: 03 Sep 2014 10:49
URI: http://etheses.whiterose.ac.uk/id/eprint/6526

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