Recent Progress of Gel Polymer Electrolytes for Sodium Sulfur Batteries

Sodium sulfur batteries (NaSBs) stand out as one of the most promising energy storage systems due to the natural abundance of raw materials, outstanding specific capacity, and excellent energy density. Yet, conventional NaSBs, which operate at high temperature (300-350 degrees C), are not applicable for daily energy storage such as batteries for mobile devices and are limited to be stationary energy storage due to their nature of extreme operating conditions and dangerous explosion hazards. On the other hand, standard aqueous room temperature NaSBs, which use liquid electrolytes, are facing many undesirable problems such as growth of sodium dendrites, short cycle life, leakage, and fire hazards. Ultimately, the sodium polysulfide shuttle effect in liquid electrolytes still remains the number one challenge in room temperature NaSBs development. Gel polymer electrolytes (GPEs) are a combination of liquid electrolytes and polymers, acting not only as electrolytes but also as a separator which can suppress current challenges and improve battery performance in room temperature NaSBs. In this review, detailed discussions are presented on physical and chemical properties of GPEs that inhibit the shuttle effect of polysulfides, unify Na+ ion transport, and prevent Na dendrite formation. Various polymer matrices for GPEs in NaSBs, such as PEO, PVDF, PVDF-HFP, PMMA, and cross-linked polymers, are discussed. Diverse fabrication techniques for GPEs in NaSBs, including solution casting, phase inversion, an in situ polymerization strategy, electrospinning, and UV curing, are explored. This review offers a prospect of possibilities for the practical application of GPEs in NaSBs with great electrochemical performance.