Solid state lithium metal batteries are largely regarded as a safer alternative to traditional lithium-ion batteries that also have higher theoretical energy densities due to the solid lithium anode. However, realizing the full potential of lithium metal batteries requires a solid electrolyte that has ionic conductivity, high lithium cation transference, and sufficient mechanical properties. This research presents the synthesis and characterization of elastomeric hyperbranched phosphonium ionenes that incorporate organic ionic plastic crystals. Elastomeric hyperbranched phosphonium ionenes were polymerized by a modified Menshutkin reaction where the phosphino groups of 1,4-bis(diphenylphosphino)butane (f = 2 monomer) were reacted with 2,2-bis(((6-bromohexanoyl)oxy)methyl)propane-1,3-diyl bis(6-bromohexanoate) (f = 4 branching monomer). Thermal analysis of these hyperbranched phosphonium ionenes showed that they have glass transition temperatures less than 0 °C and degradation temperatures of approximately 200 °C. Comparatively, hyperbranched phosphonium ionenes prepared by the A₂ + B₃ step-growth polymerization of 1,4-bis(diphenylphosphino)butane with 1,3,5-tris(bromomethyl)bromide produced covalent networks with much higher glass transition temperature (>100 °C) and degradation temperatures (>300 °C). Despite their reduced temperature stability, the elastomeric branched phosphonium ionenes showed higher ionic conductivity. This is likely due to increased segmental motion suggested by their significantly reduced glass transition temperature as measured by differential scanning calorimetry. Ongoing work on these promising solid polymer electrolytes includes doping the elastomeric branched phosphonium ionenes with organic ionic plastic crystals (OIPCs) along with lithium-based salts. OIPCs are expected to further enhance the ionic conductivity. OIPC-doped phosphonium ionenes will be characterized for their ionic conductivity, lithium transference number, and charge/discharge properties in coin cell batteries.

Nicholas Boynton

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