Polymer electrolytes for sodium-ion batteries

Sodium-ion batteries are seeing a surge in interest as a potential complementary energy storage technology in light of skyrocketing demand for lithium-ion batteries. One of the frontiers of improving sodium-ion battery competitiveness is replacing liquid electrolytes with polymer electrolytes, which contain no free-flowing solvent, to increase safety and reduce cost. Their development may one day make viable sodium-metal batteries, which would have considerable advantages in energy density. This review provides an overview of the current field of both solid-polymer and gel-polymer electrolytes for sodium-ion batteries, with a focus in the key performance parameters used to assess them. In particular, their targeted manipulation and significance for practical use are discussed. A major theme is also the interdependence of many electrochemical and mechanical properties. In addition, a quantitative comparison of hitherto reported values for these parameters across various polymer classes is undertaken for the first time.