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-αFe2+α/2(P2O7)2 (2/3 ≤ α ≤ 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.
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