Sodium-ion batteries hold promise as an enabling technology for large-scale energy storage that is safer, less expensive, and lower in terms of environmental impact than their equivalent lithium-ion batteries. In this presentation the synthesis, crystal structure, and electrochemical properties of a new sodium-ion battery cathode material, an alluaudite phase of Na_1.702Fe₃(PO₄)₃ will be described. After ball milling and carbon coating, this material exhibits a reversible capacity of 140.7 mAh/g with good cycling performance and excellent rate capability. In addition, we report a new compound in the alluaudite class, Na_0.872Fe₃(PO₄)₃, which was obtained upon desodiation of Na_1.702Fe₃(PO₄)₃. Both compounds exhibit high thermal stability. Together, these compounds represent a new cathode material for large-scale battery applications that are earth-abundant and sustainable.