Improved Electrochemical Performance of Fe-Substituted NaNi0.5Mn0.5O2 Cathode Materials for Sodium-Ion Batteries

A series of O3-phase NaFex(Ni0.5Mn0.5)(1x)O-2 (x = 0, 0.1, 0.2, 0.3, 0.4, and 1) samples with different Fe contents was prepared and investigated as high-capacity cathodic hosts of Na-ion batteries. The partial substitution of Ni and Mn with Fe in the O3-phase lattice can greatly improve the electrochemical performance and the structural stability. A NaFe0.2Mn0.4Ni0.4O2 cathode with an optimized Fe content of x = 0.2 can deliver an initial reversible capacity of 131 mAh g(1), a reversible capacity greater than 95% over 30 cycles, and a high rate capacity of 86 mAh g(1) at 10 C in a voltage range of 2.04.0 V. The structural characterizations reveal that pristine NaMn0.5Ni0.5O2 and Fe-substituted NaFe0.2Mn0.4Ni0.4O2 lattices underwent different phase transformations from P3 to P3 '' and from P3 to OP2 phases, respectively, at high voltage interval. The as-resulted OP2 phase by Fe substitution has smaller interslab distance (5.13 angstrom) than the P3 '' phase (5.72 angstrom), which suppresses the co-insertion of the solvent molecules, the electrolyte anions, or both and therefore enhances the cycling stability in the high voltage charge. This finding suggests a new strategy for creating cycle-stable transition-metal oxide cathodes for high-performance Na-ion batteries.