The first Fe-based Na + -ion cathode with two distinct types of polyanions: Fe 3 P 5 SiO 19

Herein, we report the synthesis, structure, and electrochemistry of the first Na(+)-ion cathode with two distinct types of polyanions: Fe3P5SiO19. The Fe-based cathode has a reversible capacity of ca. 70 mA h g(-1); ca. 1.7 Na(+) ions per formula can be inserted/extracted at an average voltage of 2.5 V versus Na(+)/Na.