A novel air-stable sodium iron hexacyanoferrate (R-Na1.92Fe[Fe(CN)6]) with rhombohedral structure is demonstrated to be a scalable, low-cost cathode material for sodium-ion batteries exhibiting high capacity, long cycle life, and good rate capability. The cycling mechanism of the iron redox is clarified and understood through synchrotron-based soft X-ray absorption spectroscopy, which also reveals the correlation between the physical properties and the cell performance of this novel material. More importantly, successful preparation of a dehydrated iron hexacyanoferrate with high sodium-ion concentration enables the fabrication of a discharged sodium-ion battery with a non-sodium metal anode, and the manufacturing feasibility of low cost sodium-ion batteries with existing lithium-ion battery infrastructures has been tested.

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16. We thank the entire Voyager team at NASA Headquarters and the Jet Propulsion Laboratory (JPL) for their support. We are especially grateful to R. Poynter for his invaluable assistance and support. We also thank E. Miner and J. Diner for their efforts to arrange the wideband coverage; J. Anderson, P. Jepsen, and G. Garneau for their assistance with the wideband data processing; C. Stembridge for his help in solving numerous problems; H. Bridge, J. Belcher, J. Scudder, and N. Ness for providing data in advance of publication and for their helpful discussions; and R. Anderson, R. West, L. Granroth, and R. Brechwald for carrying out the data reduction. The research at the University of Iowa was supported by NASA through contract 954013 with JPL, through grants NGL-16-001-002 and NGL-16-001-043 from NASA Headquarters, and by the Office of Naval Research. The research at TRW was supported by NASA through contract 954012 with JPL.

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