Polymer Electrolytes for Rechargeable Sodium Batteries: Review of Characterization of Bulk and Interfacial Properties

Sodium-based batteries are gaining momentum as a cost-effective, sustainable alternative to lithium-based batteries, driven by the global demand for scalable energy storage. At the same time, polymer electrolytes are being widely pursued as safer, more electrochemically stable alternatives to liquid electrolytes. Sodium polymer electrolytes require advancement in various aspects, such as ion transport and electrode compatibility, before application in rechargeable sodium-ion or sodium metal batteries. This review examines the progress in characterization of the bulk properties of sodium polymer electrolytes, molecular interactions in the bulk, and their interfaces/interphases with electrodes, with attention paid to differences between characterization methodology and select properties of sodium versus lithium analogs. Highlighted topics include ionic conductivity, sodium transference, ion speciation, electrochemical stability, safety, and electrochemical and chemical characterization of interfaces, interphases, and with sodium sulfur cathodes.