Na4‐αM2+α/2(P2O7)2 (2/3 ≤ α ≤ 7/8, M = Fe, Fe0.5Mn0.5, Mn): A Promising Sodium Ion Cathode for Na‐ion Batteries

Abstract A new polyanion‐based compound, Na 3.12 M 2.44 (P 2 O 7 ) 2 (M = Fe, Fe 0.5 Mn 0.5 , Mn) is synthesized and examined as a cathode for Na ion batteries. Off‐stoichiometric synthesis induces the formation of a Na‐rich phase, Na 3.32 Fe 2.34 (P 2 O 7 ) 2 ‐ a member of the solid solution series Na 4‐α Fe 2+α/2 (P 2 O 7 ) 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.