Solvent‐free Ternary Polymer Electrolytes with High Ionic Conductivity for Stable Sodium‐based Batteries at Room Temperature

Transitioning to solid-state batteries using polymer electrolytes results in inherently safer devices and can facilitate the use of sodium metal anodes enabling higher energy densities. In this work, solvent-free ternary polymer electrolytes based on cross-linked polyethylene oxide (PEO), sodium bis(fluorosulfonyl) imide (NaFSI) or sodium bis(trifluoromethanesulfonyl) imide (NaTFSI) and N-butyl-N-methyl-pyrrolidinium-based ionic liquids (ILs, Pyr$_{14}$FSI or Pyr$_{14}$TFSI) are developed. Synthesized polymer membranes are thoroughly characterized, verifying their good thermal and electrochemical stability, as well as a low glass transition and crystallinity, thus high segmental mobility of the polymer matrix. The latter results in good ionic conductivities around 1×10$^{−3}$ S cm$^{−1}$ at 20 °C. The polymer electrolytes are successfully employed in sodium-metal battery (SMB) cells operating at room temperature (RT) and using P2-Na$_{2/3}$Ni$_{1/3}$Mn$_{2/3}$O$_2$ layered oxide as cathode. The electrochemical performance strongly depends on the choice of anion in the conducting sodium salt and plasticizing IL. Furthermore, this solid-state SMB approach mitigates capacity fading drivers for the P2-Na$_{2/3}$Ni$_{1/3}$Mn$_{2/3}$O$_2$, resulting in high Coulombic efficiency (99.91 %) and high capacity retention (99 % after 100 cycles) with good specific capacity (140 mAh g$^{−1}$).