Influence of Salt Anions on the Reactivity of Polymer Electrolytes in All-Solid-State Sodium Batteries

Solid-state batteries have received renewed attention in recent years. The present study compares all-solid-state sodium batteries containing sodium metal anodes, NaTi₂(PO₄)₃ (NTP) cathodes and PEO polymer electrolyte (PE) with two salts—NaPF₆ and NaClO₄. Electrochemical properties were determined by means of both AC and DC measurements. For fresh symmetric cells, it seems like using NaClO₄ as the salt leads to a lower interfacial resistance, but during contact with Na anodes, PEO:NaClO₄ PE produces an unstable interface whereas PEO-NaPF₆ forms a stable interface upon contact with sodium anodes. Battery prototypes with PEO:NaClO₄ showed a better performance than those using PEO:NaPF₆ in terms of specific capacity and energy content, as NaPF₆ produces HF by-product which impairs the performance of full cells. However, the use of composite solid electrolyte containing PEO:NaPF₆ and TiO₂ nanotubes alleviates detrimental effects of HF and thus enables a better performance of SS Na battery prototypes. This is probably due to the titania particles acting as a scavenger of HF, an inevitable contaminant in electrolyte systems containing PF₆⁻ anions. On adding TiO₂ to polymer electrolytes, of both salts, the performance improves, what means that the TiO₂ has several positive effects in these systems.