Seawater batteries (SWBs) have been mostly researched for large scale energy storage and (sub-) marine applications. In a SWB, the aqueous catholyte (seawater) and a non-aqueous anolyte (aprotic solvent solution)...

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, Pyr14FSI or Pyr14TFSI) 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‐Na2/3Ni1/3Mn2/3O2 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‐Na2/3Ni1/3Mn2/3O2, resulting in high Coulombic efficiency (99.91 %) and high capacity retention (99 % after 100 cycles) with good specific capacity (140 mAh g−1).

Invited for this month's cover are the Electrochemistry for Batteries and Beyond Li: materials & interphases groups led by Prof. Stefano Passerini and Dr. Maider Zarrabeitia, respectively, at Helmholtz Institute Ulm (HIU). The cover picture shows a solvent‐free polymer electrolyte‐based sodium‐metal battery operating at room temperature for stationary applications. Read the full text of the Research Article at 10.1002/batt.202300092.