Safe polymer electrolytes and high performing anode materials for Na-based secondary batteries

Secondary (rechargeable) Na-based batteries are an attractive alternative to Li-ion batteries for large/scale energy storage technologies, because of high-energy density, sodium abundance, low-cost, simple design, and easiness in maintenance. However, safety concerns related to the use of carbonate-based liquid electrolytes (toxic and volatile) are similar to their Li-based counterpart, mainly due to their flammability and risk of explosion. The most striking solution at present is switching to an all solid-state design exploiting polymer electrolyte materials, ceramics, and hybrids thereof. Moreover, the use of an appropriate and efficient negative electrode material to replace the unpractical graphite-based anodes is fundamental to obtain high energy density batteries. In the present work, an overview will be provided on both truly solid and quasi-solid polymer electrolytes specifically conceived and developed for Na-ion cells, based on polyethylene oxide (PEO), acrylates/methacrylates and/or mixtures thereof. Eventually, pyranose ring based natural additives and/or low volatile plasticizers are added along with supporting sodium salts to improve specifically defined characteristics. Both standard casting and UV-induced photopolymerization techniques have been explored. Furthermore, our results regarding nanostructured TiO2 nanotubular electrodes will be presented, including different electrochemical responses of amorphous, rutile and anatase based nanotube arrays, obtained by simple anodic oxidation, when tested as binder- and conducting additive-free electrodes in lab-scale sodium cells. Thorough modelling by ab-initio and DFT calculations of the sodiation processes will be also discussed.