The transition towards solid sodium batteries: easily processable electrodes and electrolytes

In recent years, large-scale energy storage systems are becoming extremely important to realize the load levelling of intermittent renewable energy sources, such as wind and solar, into the grid. Secondary (rechargeable) sodium-based batteries may represent the key enabling technology in this respect, because of high-energy density, low-cost, simple design, and easiness in maintenance. However, currently studied materials and processes are not in line with a truly sustainable point of view. Here we offer an overview of our recent developments on innovative polymer electrolytes for sodium-ion batteries. In our labs, we develop different kind of polymer electrolytes by means of different techniques, including simple solvent casting and UV-induced photopolymerization (UV-curing), being simple, low-cost and easily scalable to an industrial level. Besides, our research focuses on new sustainable polymers such as cellulose or linear polyglycidols, that could be a solution in future applications. All samples were thoroughly characterized from the physico-chemical and electrochemical viewpoints. They exhibited excellent ionic conductivity and wide electrochemical stability window, which ensure safe operation even at ambient conditions. Electrochemical performances in lab-scale devices were evaluated by means of cyclic voltammetry and galvanostatic charge/discharge cycling exploiting different electrode materials. Our benchmark solid polymer electrolyte, when assembled in TiO2-based lab-scale sodium cells, delivered a stable specific capacity of about 250 mAh g−1 at ambient temperature upon constant current cycling at 0.1 mA cm−2. Its intrinsic stability was also confirmed by very long-term cycling test that exceeded 5200 h of continuous operation, which is definitely remarkable for a truly quasi-solid system.