Innovative polymer electrolytes for safe, low-cost and durable sodium-ion batteries

In the recent years, large-scale and high-energy storage systems are becoming extremely important to realize the load leveling of intermittent renewable energy sources, such as wind and solar, into the grid. Secondary (rechargeable) sodium-based batteries represent the most promising technology in this respect, because of their high-energy density, low-cost, simple design, and easiness in maintenance. However, standard batteries use liquid electrolytes as ion transport media; these are based on toxic and volatile organic carbonate solvents, and their flammability clearly raises safety concerns. The most striking solution at present is to switch on all solid-state designs exploiting polymer materials, films, ceramics, etc. Here, we offer an overview of our recent developments on innovative polymer electrolytes for sodium-ion batteries. Polymer electrolytes were prepared through different techniques, including simple solvent casting and UV-induced photopolymerization (UV-curing), being simple, low-cost and easily scalable to an industrial level. All samples were thoroughly characterized in the physico-chemical and electrochemical viewpoint. They exhibited excellent ionic conductivity and wide electrochemical stability window, which ensure safe operation 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 (prepared by water-based procedures exploiting green carboxymethylcellulose as binder). Work on Na-ion polymer batteries for moderate temperature application is at an early stage, only lab-scale cells were demonstrated so far. Nevertheless, with the appropriate choice and optimisation of electrode/electrolyte materials (and successful combination thereof), the intriguing characteristics of the newly developed polymer electrolytes here presented postulates the possibility of their effective implementation in safe, durable and high energy density secondary Na-based solid-state devices conceived for green-grid storage and operating at ambient and/or sub-ambient temperatures.