Synthesis of high molecular flexibility polycarbonates for solid polymer electrolytes

• A new polycarbonate was synthesized and showed a record-low Tg of −49.4 °C.
• Crosslinking provided mechanically stable polymer electrolyte films.
• Ionic conductivity up to 2 × 10−7 S cm−1 at room temperature was demonstrated.

A new self-plasticizing aliphatic polycarbonate comprising flexible alkyl and alkyl ether side groups was designed and synthesized from six-membered cyclic carbonate monomers with the aim of producing a material with high molecular flexibility (low Tg) and concomitant high ionic conductivity when used as a polymer electrolyte. The Tg of the novel polycarbonate was determined to be −49.4 °C at a molecular weight of 34 400 g mol−1, which is the lowest reported Tg to date for a substituted poly(trimethylene carbonate). UV-crosslinked polymer electrolytes were produced based on this novel material together with LiTFSI salt and showed ionic conductivities in the range of 2 × 10−8 to 2 × 10−7 S cm−1 at room temperature and 1 × 10−6 to 1 × 10−5 S cm−1 at 100 °C. The limited ionic conductivities of these electrolytes indicate that high molecular flexibility alone does not guarantee fast ion transport in solid polymer electrolytes and that other factors, such as the polarity of the polymer host material, will also influence the transport properties of the electrolyte.

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