Fe-rich pyrophosphate with prolonged high-voltage-plateaus and suppressed voltage decay as sodium-ion battery cathode

Fe-based polyanionic materials are potential cathode candidates for sodium-ion batteries, however, voltage decay and cycling stability become main challenges for practical applications. Besides, the mechanism of phase transition during charge/discharge processes is still unclear. Herein, the Fe-rich phase Na1.4Fe1.3P2O7 with Na/Fe atom vacancies is constructed, showing robust voltage decay suppression and a larger ratio of the plateau length (6:1) at 3.0 and 2.5 V than that of the conventional Na-rich phase Na2FeP2O7 (2.5:1). After 650 cycles at 1 C, the capacity and average voltage retentions of Fe-rich phase are 84 % and 95 %, respectively, much higher than that of Na-rich phase (12 % and 61 %). Furthermore, Mössbauer spectra unveil that, during discharge processes at high voltages, more Fe3+reduction in Fe-rich phase is the intrinsic reason for larger plateau length and lower voltage decay. Density functional theory (DFT) calculation results demonstrate that the higher reduction ability of Fe3+ in Fe-rich phase at high voltages is due to the better reversibility of the two-phase transition between β-NaFeP2O7 and Fe-rich phase than that between β-NaFeP2O7 and Na-rich phase. These allow Fe-rich phase with higher energy densities, lower voltage decay and better cycling stability at high voltage plateaus. Our work provides a new idea for safe design, low-cost and long cycle life sodium ion cathode battery materials.