Organic Ionic Plastic Crystals as solid electrolytes for lithium batteries
thesisposted on 27.02.2017 by Jin, Liyu
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Organic Ionic Plastic Crystals (OIPCs) are increasingly drawing attention as a new type of solid-state electrolyte for lithium rechargeable batteries, because they can essentially relieve the safety concerns of the present commercial lithium batteries based on organic and flammable solvents and thus pave the way towards more robust energy storage solutions for electric vehicles and power grids. However, although significant progress has been made towards the realisation of practical OIPC-based solid electrolytes, the lack of thorough fundamental understanding of their properties, especially the nature of their ionic motion and transport and the effects of Li salt addition, has limited the development of prototype lithium cells using OIPC-based electrolytes. This PhD thesis therefore has three goals: 1) to obtain more insights into the fundamentals of the materials, especially their phase-dependent plastic crystal behaviour; 2) to under- stand the effects of the addition of Li ions on the structure and dynamics of OIPC matrices; 3) to demonstrate prototype Li cells with OIPC-based electrolytes operating at ambient temperature. For Goal 1, two recently synthesised phosphonium cation-based OIPCs, i. e. diethyl(methyl)(isobutyl)phosphonium hexafluorophosphate ([P1224][PF6]) and triethyl(methyl)phosphonium bis(fluorosulfonyl)imide ([P1222][FSI]) were investigated by a combination of analytical methods and theoretical simulations/calculations. The evolution of the motions of the cations and the anions in different phases of both plastic crystals was examined, and models proposed to explain the strongly phase-dependent ionic conductivities of these OIPCs. Cooperative motion of cations and anions was proposed for [P1222][FSI] at a particular temperature range, in which some unusual phase behaviour was observed. For Goal 2, molecular dynamics simulations of the effects of lithium salt addition to a model plastic crystal, tetramethylammonium dicyanamide ([N1111][DCA]) were performed. Cluster formation between Li ions and anions was found: Li-DCA clusters enhance the mobility of the ions that are not in the clusters. This is a somewhat surprising atomic-level explanation of how doped Li ions in the OIPC enhance its ion transport properties. For Goal 3, the electrochemical properties of triisobutyl(methyl) phosphonium bis(fluorosulfonyl) imide ([P1444][FSI]) were examined, and this plastic crystal was used to build prototypical Li metal cells. These cells showed, for the first time, practical cell performance (ca. 160 mA h g-1 discharge capacity achieved at 0.1 C) at ambient temperature. The main contributions of this thesis to the field are 1) proposing new models of the ionic motions of OIPCs with respect to their phase behaviour 2) showing the value of the combined analytical and theoretical methodology in studying OIPCs; 3) demonstrating promising Li cell performance with an OIPC-based electrolyte at ambient temperature.