The rapid widespread of electric vehicles, smart grids and portable electronics is expected to require a large number of energy storage devices that Li-ion batteries alone will not be able to fulfill. In this scenario, a quest for innovative electrochemical energy storage technologies has begun [1]. Sodium secondary batteries (NaBs) are among the best solutions due to: (a) the high abundance and low cost of raw materials; (b) the low sodium standard reduction potential; and (c) the analogies between Na and Li chemistries, thus facilitating the implementation of the former. The state-of-the-art electrolytes for the reversible deposition of sodium are based on organic solvents, which are highly flammable and not stable towards sodium metal. Therefore, research activities in this field are mainly focused on the development of safe and high-performing Na⁺ ion conducting solid-state electrolytes.
Inspired by the pioneering work done by Di Noto and co-workers [2-4], herein we present a family of hybrid inorganic-organic polymer electrolytes (HIOPEs) for advanced solid-state NaBs. The HIOPE is synthetized reacting zirconium ethoxide with polyethylene glycol (PEG400) and sodium perchlorate. As a result, a 3D network is obtained, where the inorganic zirconium metal nodes are interconnected by means of organic PEO chains, which ensure flexibility to the overall structure. In addition, positively charged Zr nodes partially coordinate perchlorate anions, thus raising the sodium transference number. The flexibility and ion conductivity of the HIOPE is further boosted doping with the poly(ethylene glycol) dimethyl ether (PEGDME250) plasticizer, achieving a room temperature value higher than 10^-4 S cm^-1. The proposed solid-state electrolytes are studied in terms of: (a) thermal and structural properties, with a particular focus on the interactions established between the different chemical species and complexes composing the HIOPEs; and (b) conduction mechanism, elucidated by means of broadband electrical spectroscopy in a wide range of temperatures and frequencies. Taking all together, this work offers a deep insight on the application of non-traditional solid state electrochemical functional components for the application in NaBs.