Model Compounds Based on Cyclotriphosphazene and Hexaphenylbenzene with Tethered Li+-Solvents and Their Ion-Conducting Properties

To improve the lithium ion conductivities of currently used electrolytes, it is critical to understand how the transport of the lithium ions within the matrix is influenced by their interactions with solvating moieties. Therefore, well-defined model compounds based on cyclotriphosphazene (CTP) and hexaphenylbenzene (HPB) cores were prepared, bearing side groups containing the structural element of ethylene carbonate, which is the common solvent for lithium salts used as electrolytes in Li-ion batteries. All model compounds were highly pure and thermally stable up to at least 250 degrees C, covering a broad range of glass transition temperatures from -79 degrees C up to +3.5 degrees C. The temperature-dependent ionic conductivities of the blends follow a William-Landel-Ferry (WLF) type behavior with the corresponding glass transition temperatures as reference. Though the glass transition temperatures of the blends are low, their conductivities are only in the range of typical polymer electrolytes, implying that the coordination between the cyclic carbonate functionality and the Li-ion is apparently too tight to allow for fast Li-ion dynamics.