Na4-M2+/2(P2O7)2 (2/3 7/8, M = Fe, Fe0.5Mn0.5, Mn): A Promising Sodium Ion Cathode for Na-ion Batteries

A new polyanion-based compound, Na3.12M2.44(P2O7)2 (M = Fe, Fe0.5Mn0.5, Mn) is synthesized and examined as a cathode for Na ion batteries. Off-stoichiometric synthesis induces the formation of a Na-rich phase, Na3.32Fe2.34(P2O7)2 - a member of the solid solution series Na4-Fe2+/2(P2O7)2 (2/3 7/8) - which delivers a reversible capacity of about 85 mA h g-1 at ca. 3 V vs. Na/Na+ and exhibits very stable cycle performance. Above all, it shows fast kinetics for Na ions, delivering an almost constant 72% reversible capacity at rates between C/10 and 10C without the necessity for nanosizing or carbon coating. We attribute this to the spacious channel size along the a-axis, along with a single phase transformation upon de/sodiation.