Sodium-based polymer electrolytes for sodium-ion battery application

The research in the lithium ion batteries field is noticing the emerging problems related to safety conditions in energy storage devices. This important drawback has focused the research to the development of batteries that using alternative metals such as sodium. Sodium metal is a valid alternative to lithium as a negative electrode, due its low price, natural abundance, no toxicity, low atomic weight and high reduction potential (-2.71 V vs SHE). The combination of its high reduction potential and its low atomic weight could allow its use as an anodic material in rechargeable batteries for high energy applications. Sodium, coupled to proper electropositive materials (cathode) allows to obtain cell potential higher than 2 V. It is necessary to develop novel electrolytes to improve the characteristics of the new sodium-based technologies. In this perspective, we have addressed our research to the synthesis and characterization of polymer electrolytes based on poly(ethylene oxide) (P(EO)) complexed with sodium salts [1]. For this purpose, binary polymer electrolyte membranes of P(EO) 600.000 MW complexed with sodium (bis)trifluoromethanesulfonate imide salt (Na(CF3SO2)2N namely, NaTFSI) [2] were synthesized by hot-pressing technique (solvent-free synthetic route). in different EO:NaTFSI ratios. All the membranes showed good ionic conductivities (σ= 10-3 S/cm at 60°C). However, the rich NaTFSI samples demonstrated sticky features due to the anion-polymer backbone interactions [3]. The DSC analyses revealed that the melting point of the membranes decreased by increasing the NaTFSI concentration. The best performances in terms of conductivity, thermal and mechanical properties were showed by the P(EO)20:NaTFSI sample. The addition of nanostructured SiO2 to the P(EO)20:NaTFSI membranes enhanced the mechanical properties. Only the addition of the 5wt.% of the ceramic filler allowed to increase its conductivity (1.3 x 10-3 S/cm at 80°C). The stability window of the Na/P(EO)20NaTFSI/Super P cell is extended up to 4.3 V vs Na/Na+. In a symmetrical Na/polymer electrolyte/Na cell, the interface resistance increased by about 20% after 30 days. In comparison to the ceramic-based membranes revealed a more stable interface.