Mn-doped Na4Fe3(PO4)2(P2O7) facilitating Na+ migration at low temperature as a high performance cathode material of sodium ion batteries

Abstract

Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 (NFPP) is known as a cathode material with great potential for sodium ion batteries (SIBs) due to its thermodynamic stability, considerable theoretical capacity and small volume change. However, its inherent poor conductivity leads to low discharge capacity and poor cycle stability, which greatly limits its widespread and practical application. In this work, a Mn 2+ doped NFPP together with the graphene modification, is proposed. It is found that the optimal Na 4 Fe 2.7 Mn 0.3 (PO 4 ) 2 P 2 O 7 /rGO(Mn0.3-NFPP/rGO) can provide an initial discharge capacity of 131.5 mAh g −1 at 0.1C, (1C = 129mAh g −1 ) also an excellent rate performance (70.2 mAh g −1 at 50C) and good long cycle stability (97.2% capacity retention after 2000 cycles at 10C) can be obtained. Impressively, the as-prepared Mn0.3-NFPP/rGO also shows excellent low-temperature performance, at −20 °C, it demonstrates a discharge capacity of 85.3 mAh g −1 at 0.2C and good rate performance. Density functional theory (DFT) calculation shows that Mn 2+ doping reduces the Na + migration energy barrier, and also narrows the bandgap of the NFPP lattice, which is beneficial to improve the Na + diffusion kinetics and conductivity. In addition, the doping of Mn 2+ can further help to improve its structural stability during the long cycling process. • Mn-doped Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) together with graphene modification is proposed. • Mn 2+ doping reduces the Na + migration energy barrier. • Mn 2+ doping narrows the bandgap of Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ). • The Na + migration is facilitated by Mn 2+ doping, particularly at low temperature.