Lanthanum-doped Na4Fe3(PO4)2P2O7/C as cathode materials in sodium-ion batteries: Enhanced ion diffusion kinetics and embedded pseudocapacitance

The iron-based hybrid polyanionic compound Na4Fe3(PO4)2(P2O7) (NFPP) is perceived as a potential cathode for sodium-ion batteries (SIBs) on account of its high theoretical capacity, resource abundance, nontoxicity and stable framework structure, however, its sodium storage capabilities are hindered by sluggish ionic mobility kinetics. This paper reports the successful synthesis of porous carbon-modified La-doped Na4Fe3(PO4)2(P2O7) materials using sol-gel and freeze-drying techniques. Na4Fe2.94La0.04(PO4)2(P2O7)/C (NFPP-0.04) showcases remarkable sodium ion mobility, outstanding cycling stability, and exceptionally high discharge specific capacity. These attributes are attributed to the enhancement of embedded pseudocapacitance and electrochemical kinetics through the synergistic effect of lanthanum doping and freeze-drying technology. The NFPP-0.04 achieves a high discharge capacity (128.4 mAh g- 1 at 0.2 C, 1 C = 129 mAh g- 1) and eminent durability (capacity retention of 97.2 % after 100 cycles at 0.2 C). Furthermore, in situ XRD analysis revealed the exceptional structural stability of NFPP-0.04. Additionally, the correlation of cyclic voltammetry (CV) curves at various scan rates confirms that the charge/discharge mechanism of NFPP-0.04 predominantly relies on embedded pseudo- capacitance. This study highlights the feasibility of enhancing the sodium storage kinetics of materials through precise ion doping. Consequently, NFPP-0.04 emerges as a promising candidate for cathodes in sodium-ion batteries (SIBs).