(Invited) Solid State Hybrid Inorganic Organic Polymer Electrolytes for Advanced Sodium Secondary Batteries

The large-scale rollout of electric vehicles, smart grids and portable electronics is expected to require an amount of energy storage devices that the Li-ion technology alone will not be able to satisfy. In this concern, a quest for novel electrochemical energy storage technologies has started [1]. Sodium secondary batteries appear to be a good choice due to: (i) the high availability of raw materials; (ii) the low cost of sodium; (iii) the low sodium standard reduction potential; and (iv) the similarity between the chemistries of sodium and lithium, which facilitates the transition between the two technologies. Up to date, the best-performing electrolytes for the reversible deposition of sodium are based on organic solvents, which suffer from safety concerns and a poor stability towards sodium metal. Thus, research activities in this field are devoted to the development of safe and stable solid state electrolytes able to efficiently transport Na + ions. 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 sodium secondary batteries. The initial HIOPE is obtained by means of a reaction between zirconium ethoxide and polyethylene glycol (PEG400). The resulting material is doped with sodium perchlorate as source of Na + ions. In this system a 3D network is obtained, where inorganic zirconium metal nodes are interconnected by means of organic PEO chains. The latter ensure flexibility to the overall structure. Na + ions are coordinated by and exchanged between the oxygen atoms of the ethereal functionalities of PEO chains. In addition, to guarantee a good structural stability, positively charged Zr nodes partially coordinate perchlorate anions, thus raising the sodium transference number. After doping with the poly(ethylene glycol) dimethyl ether (PEGDME250) plasticizer, a room temperature conductivity higher than 10 -4 S cm -1 is demonstrated. An advanced study of the thermal and structural properties of the proposed materials is presented, with a particular focus on the interactions established between the different chemical species and complexes composing the HIOPEs. The conduction mechanism is elucidated starting from the results obtained in a wide range of temperatures from broadband electrical spectroscopy studies. Taking all together, this study offers insights on the application of non-traditional solid state electrochemical functional components as replacements for conventional solvents in the emerging field of sodium secondary batteries. Acknowledgments The project “Interplay between structure, properties, relaxations and conductivity mechanism in new electrolytes for secondary Magnesium batteries” (Grant Agreement W911NF-21-1-0347-(78622-CH-INT)) of the U.S. Army Research Office. The project “ACHILLES” (prot. BIRD219831) of the University of Padua. The project “VIDICAT” (Grant Agreement 829145) of the FET-Open call of Horizion 2020. The project TRUST (protocol 2017MCEEY4) of the Italian MIUR funded in the framework of “PRIN 2017” call. References [1] R. Dominko, J. Bitenc, R. Berthelot, M. Gauthier, G. Pagot and V. Di Noto, , J. Power Sourc . 478 (2020) 229027. [2] V. Di Noto, V. Zago, S. Biscazzo and M. Vittadello, Electrochim. Acta 48 (2003) 541-554. [3] M. Jeyapandian, S. Lavina, S. Thayumanasundaram, H. Ohno, E. Negro and V. Di Noto, J. Power Sourc . 195 (2010) 341-353. [4] V. Münchow, V. Di Noto and E. Tondello, Electrochim. Acta 45 (2000) 1211-1221.