Abstract:
The objective of this research project was synthesis of polymer gel electrolytes with the properties required for use in rechargeable lithium ion batteries. The target properties were ionic conductivity of the order 1 mS cm⁻¹, sufficient rigidity to allow utilisation in rechargeable lithium ion batteries, and high Li⁺ ion transference number. The approach that was adopted to achieve the target properties was synthesis of semiinterpenetrating networks (SIPN) comprising crosslinked poly(methyl methacrylate) (PMMA) gel swollen with N, N-dimethyl acetamide (DMAA), and charge-carrying linear chains of the lithium salt of 2-acrylamido-2-methylpropanesulphonic acid (LiAMPS). Because the negative charges (sulphonate anions) were covalently bonded to the poly(LiAMPS) chains whose translational and flexural mobility was limited by the surrounding PMMA network, the Li⁺ counterions were the most mobile species in the system. Hence this system approximated a single ion conducting polyelectrolyte. The rigidity of the gel, on the other hand, was provided primarily by the PMMA network; hence charge transport and mechanical properties were partially decoupled in the SIPN gel. That characteristic of the system allowed the charge transport and mechanical properties of the gel to be varied independently, to some extent. It was envisaged that there were two predominant modes of charge transport, via Li⁺ ion migration in the SIPN gels, namely (i) an intramolecular mechanism in which Li⁺ ions jump from (negatively charged) site to site along the linear chains; (ii) an intermolecular mechanism in which the cations jump between negatively charged sites on adjacent chains when flexure of adjacent chains allows that to occur. The effect of variation of the composition of both the linear chains and the network was investigated in an attempt to understand the relative importance of those modes of charge transport. The general procedure for synthesising the SIPN gels was to first polymerise the linear chains by free radical initiation in DMAA, then add the network forming monomer (or monomers) together with crosslinking monomer (ethyleneglycol dimethacrylate) and further free radical initiator (azobis (cyclohexanecarbonitrile)) and heat for an appropriate time to establish the network. Gels were formed in a glass mould that produced sheets about 2 mm thick. Four series of SIPN polyelectrolyte gels and a series of plasticized polyelectrolyte membranes were synthesized and characterized by their conductivity and shear storage modulus (as a measure of gel stiffness). In the first series gels were synthesized from methyl methacrylate and copolymers of LiAMPS with four different comonomers: styrene, vinyl acetate, VeoVa 10 and sodium styrene sulphonate. The electrical conductivity and modulus of each gel was determined as a function of comonomer/LiAMPS ratio. The gels based on linear chains of poly(LiAMPS-costyrene), in particular, showed a good balance of conductivity and stiffness. The highest conductivity (1.85 mS cm⁻¹ at 25°C) was found for a styrene/LiAMPS ratio=0.8. Since it had been established that the presence of fluorine increases the conductivity and strength of most polymers, a second type of SIPN polyelectrolyte gel was synthesized using fluoroacrylates and fluorostyrene. In these gels the linear chains were poly(LiAMPS-cofluorostyrene), and the monomer from which the network was constructed was hexafluoroisopropyl acrylate (HIPA) and/or tetrafluoroethyl acrylate (TEFA) in the presence or absence of MMA. The series of gels with linear chains containing varying proportions of LiAMPS and fluorostyrene showed strong dependence of conductivity on the proportion of fluorostyrene. The stiffest gels with reasonably high conductivity were obtained by substituting HIPA or TFEA for MMA. However the conductivity was enhanced in gels that contained both MMA and TFEA. Another type of SIPN polyelectrolyte gel was synthesized by using LiAMPS grafted ethyl cellulose (EC) as the polyelectrolyte component. Gels with both acceptable conductivity (~1 mS cm⁻¹) and relatively high modulus were obtained by first grafting linear chains of LiAMPS on EC backbones. The graft copolymers were characterised by infrared spectroscopy (FTIR), X-ray powder diffraction (XRD), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). SIPN polyelectrolyte gels were produced by forming a crosslinked PMMA network in a solution of EC-graft-poly(LiAMPS). The relative proportions of EC-graftpoly( LiAMPS), MMA and EGDMA were varied. In a variation of this work, EC was used as a component of polyelectrolyte gels in which the other components were poly(LiAMPS-co-styrene) linear chains and crosslinked PMMA. Three series of these gels were synthesized, with (a) varying amounts of styrene; (b) increased EGDMA concentration; (c) reduced MMA content. In a further variation of the work with EC, a series of plasticized polyelectrolyte membranes were synthesized and their conductivity determined. Linear chains of poly(LiAMPS-co-styrene) were synthesised in a solution of EC in N, N-dimethyl acetamide (DMAA). Azobis (cyclohexanecarbonitrile) (ABCN) was used as an initiator. The chains were then crosslinked with EGDMA in the presence of diisooctyl phthalate (DIOP), and the solvent was evaporated to form solid polymer membranes.