Cross-Linked PEG Networks as Flexible Electrolytes for Solid-State Sodium Batteries: Ionic Transport, Long-Term Stability and Life Cycle Assessment

Solid-state sodium batteries based on polymer electrolytes offer a sustainable solution to overcome current and near-future needs regarding the growing energy and transport electrification issues. In this work, we propose the development of solvent-free polymer electrolytes based on an unsaturated polyether, which, once cross-linked, leads to an amorphous structure at room temperature that favors ionic transport towards reliable and robust solid-state sodium batteries operative at moderate temperatures. Using NaClO4 and NaPF6 as sodium salts, the best polymer electrolyte reaches an ionic conductivity in the range of 0.02 mS·cm−1 (30 °C)–0.90 mS·cm−1 (100 °C) with a lifetime superior to 2000 h after plating and stripping. Regarding electrochemical performance, a maximum specific capacity of 110.2 mAh·g−1 (C/20) is obtained for the polymer electrolyte including NaClO4, using Na and C/FePO4 as anode and cathode, respectively, which represents about 65% of the theoretical value expected for FePO4. In view of more sustainable energy storage devices, a life cycle assessment is also applied. While the polymer matrix is identified as the main environmental hotspot, the choice of Na salt significantly affects the overall impact, with NaClO4 exhibiting lower climate change and particulate matter impacts than NaPF6.