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

Secondary (rechargeable) sodium-based batteries are an attractive alternative to lithium-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 is fundamental in order to obtain high energy density batteries, and the current graphite-based materials used in Li-ion batteries cannot be used for the sodium counterpart. 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 secondary 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 novel nanostructured TiO2 nanotube negative electrodes, will be presented. The different electrochemical responses are presented between amorphous, rutile and anatase based TiO2 nanotubular arrays, obtained by simple anodic oxidation, when tested as binder- and conducting additive-free electrodes in lab-scale sodium cells. Thorough modelling of the sodiation processes is also envisaged.