Monoclinic Sodium Iron Hexacyanoferrate Cathode and Non-Flammable Glyme-Based Electrolyte for Inexpensive Sodium-Ion Batteries

One of the key requirements of large-scale grid-storage systems is development of inexpensive and safe batteries. Sodium-ion batteries using earth-abundant Fe or Ti based cathodes and anodes would be ideal candidates for such storage systems. Herein, a new phase of Na-rich and all Fe Prussian Blue Analogue, monoclinic Na2Fe2(CN)6.2H2O, is reported as a potential cathode for such grid-storage sodium-ion batteries. This water-insoluble and air-stable cathode can deliver 85 mAh g−1 at an average discharge voltage of 3 V vs Na/Na+ with excellent cycle life (3,000 cycles). Many facets about its sodium storage characteristics are discussed with particular emphasis on the role of interstitial water on the sodium storage performance and its conversion to the dehydrated rhombohedral phase. Its compatibility with a newly developed non-flammable glyme-based liquid electrolyte, 1M NaBF4 in tetraglyme, is also disclosed along with general electrochemical and thermal characterization of this electrolyte for sodium-ion battery application. Finally, three different types of full cells are revealed with either monoclinic or rhombohedral phase as cathode and graphite or the recently reported Na2Ti3O7 ⇋ Na3-xTi3O7 pathway of Na2Ti3O7 as anode. Full cell energy densities of 70–90 Wh kg−1 (using cumulative cathode and anode weights) could be obtained without any pre-cycling steps. This new cathode and safe electrolyte may hold great promise toward development of inexpensive, non-flammable and highly stable grid-storage sodium-ion batteries.