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

Abstract

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.