Characterization of NaX (X: TFSI, FSI) – PEO based solid polymer electrolytes for sodium batteries

Solid polymers electrolytes (SPEs) based on sodium bis(fluorosulfonyl) imide (NaFSI) and poly(ethylene oxide) (PEO) with different ether oxygen to sodium (O:Na) molar ratios (n), resulting in NaFSI(PEO)n materials are here presented for the first time. These SPEs are extensively compared with the corresponding NaTFSI(PEO)n system in terms of ionic conductivities, thermal properties, and charge carriers – to in detail outline both the role of the different anions used and the salt concentrations employed. While for the most dilute systems (n = 20) the two SPE families show similar ionic conductivities in the entire temperature range investigated (273-343 K), for n = 6 and n = 9 they differ significantly; at room temperature, the NaFSI based SPEs show lower ionic conductivities than the NaTFSI based analogues. This difference is mainly ascribed to differences in the morphology; while the NaTFSI salt, possibly by virtue of its large TFSI anion, acts to inhibit crystallization, NaFSI rather seems to favor crystallization. Furthermore, careful Raman spectroscopy analysis of the charge carrier speciation reveal higher aggregates to be present in the most concentrated SPE, NaFSI(PEO)6, and the NaFSI based SPEs in general to result in less “free” anions than the NaTFSI based SPEs. Moreover, as both NaTFSI(PEO)n and NaFSI(PEO)n for n = 20 and n = 9 exhibit very similar glass transition temperatures, the FSI ion seem to be equally plasticizing as the TFSI ion, but for n = 6 the different speciation in terms of charge carriers also affects the relative dynamics of the polymer chains.