Crosslinked Hyperbranched Polyglycerol-Based Polymer Electrolytes for Lithium Metal Batteries

Tailored partially methylated and methacrylated hyperbranched polyglycerols (hbPG-MAx/OMey) combined with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) as conducting salt were investigated after crosslinking with respect to their application as solid polymer electrolytes (SPE) in lithium metal batteries. For sample preparation and coating, a straightforward solvent-free photopolymerization method was applied. With the aim of finding the right balance between mechanical and electrochemical properties, electrolytes with different crosslinking densities were studied. High crosslink density increases mechanical integrity but reduces local chain motion and thus ionic conductivity at the same time. Differential scanning calorimetry (DSC), chronoamperometric and impedance measurements show that the hyperbranched polyether structure interacts strongly with lithium cations. Finally, the SPE with the lowest crosslinking density was selected and investigated in cycling tests due to the parameters of highest absolute values in conductivity (2.1 × 10−6 S cm−1 at 30 °C; 2.0 × 10−5 S cm−1 at 60 °C), lowest Tg (from DSC: −39 °C), electrochemical stability window (4.3 V vs. Li/Li+) and mechanical strength (1.6 ± 0.4 MPa at 25 °C). At low C-rates and elevated temperatures (60 °C), cells were cycled with high Coulombic efficiency. At high C-rates, a distinct decrease in specific capacity was observed due to insufficient ionic conductivity.