Lithium Ion Conductive Membranes Based on Co-continuous Polymer Blends

Abstract: There is a growing need for multifunctional polymeric materials for the development of several important energy conversion technologies. For example, the polymer electrolyte is a key component in lithium polymer batteries. The basic functions of this electrolyte are to efficiently conduct the lithium ions and physically separate the electrodes. Consequently, these electrolytes should ideally possess the ionic conductivity of a liquid while retaining the mechanical stability of a solid. In this thesis the concept of using polymer blends to obtain electrolyte membranes which combine ion conductivity and mechanical stability has been explored. The work comprises the preparation, characterisation and properties of three different lithium ion conductive membranes based on co-continuous polymer blend systems. The blend systems were all prepared from two polymeric components which were doped with a lithium salt. The main component was a salt-dissolving polymethacrylate network grafted with polyether or polyethercarbonate side chains. Two different kinds of methacrylate macromonomers were used in order to build up the graft copolymer structures. In two of the studied membrane types, poly(ethylene glycol) methacrylate macromonomers were used. In the third type, new polymeric building blocks, i.e., poly(ethylene carbonate-co-ethylene oxide) methacrylate macromonomers, were successfully synthesised via the anionic ring-opening polymerisation of ethylene carbonate. The macromonomers carried 30 mol% carbonate units in their structure. The minor component of the blends was a linear mechanically stable thermoplastic polymer which provided the dimensional stability. Poly(vinylidene fluoride-co-hexafluoropropylene) and poly(methyl methacrylate) were both investigated in this function. The electrolyte membranes were prepared by a two-step procedure, beginning with the solution casting of films of macromonomer, thermoplastic polymer, lithium salt and UV-activator. The methacrylate macromonomers were subsequently polymerised in-situ by UV-irradiation. The membranes were characterised by electron microscopy techniques, differential scanning calorimetry, Fourier transformation IR-spectroscopy, dynamic mechanical analysis, and electrochemical impedance spectroscopy to investigate the chemical composition, morphology, and the thermal, mechanical and conductive properties The membranes exhibited different phase separated morphologies, which depended on the level of salt content and crosslinking, as well as on the nature of the blend components. The strategy of blending polymeric materials to combine ionic conductivity with dimensional stability proved effective. The solid polymer electrolytes reached conductivities just above 10-5 S?cm-1 at room temperature while exhibiting a satisfying mechanical stability. Subsequent gelling of the solid membranes, by incorporating a liquid electrolyte of lithium salt dissolved in gamma-butyrolactone, gave elastic polymer gel electrolytes which reached conductivities of 10-3 S?cm-1 at room temperature. The studied membranes may potentially be used as electrolytes in different electrochemical devices such as lithium polymer batteries, electrochromic windows, and sensors. Applications which require different levels of, e.g., ionic conductivity, mechanical properties, and optical clarity, can take advantage of the quick and straight-forward membrane preparation process. It offers a platform to tailor membranes for different applications, including ion conductive membranes as well as other multifunctional materials.