Hetero-symbiotic Na3.12Fe2.44(P2O7)2-Na4Fe3(PO4)2(P2O7)/reduced graphene oxide for superior electrochemical sodium storage

Abstract

Iron-based pyrophosphates show most promising for sodium-ion batteries due to their electrochemical activity, inexpensive cost and high structural stability. However, poor intrinsic conductivity and the spontaneous formation of inactive triphylite NaFePO4 cause sluggish desodiation/sodiation kinetics and low specific capacity. Herein, a reduced graphene oxide (rGO)-coated Na3.12Fe2.44(P2O7)2-Na4Fe3(PO4)2(P2O7) (NFPO-NFPP) symbiotic heterostructure was successfully synthesized through a facile solid-phase procedure. The two hetero-symbiotic electrochemically active phases formed in the composite effectively inhibit inactive component formation and enhance the reversible sodium-extraction/insertion capacity. The three-dimensional porous structure boosts electrolyte infiltration and shorts the diffusion distance of sodium ions, and the uniform coating and the connection with rGO sheets of active particles effectively enhance electron migration kinetics, resulting in significantly enhanced electrochemical activities, sodium-ion extraction/insertion kinetics and structural stability. Benefitting from the synergistic effect of hetero-symbiotic structure and rGO incorporation, the as-synthesized NFPO-NFPP/rGO composite delivers a high reversible capacity of 107.2 mAh g-1 at 0.5C, excellent high-rate capability (93.3 mAh g-1 at 20C), and superior cycling stability. The as-assembled NFPO-NFPP/rGO||hard carbon full cells also deliver excellent electrochemical properties and stable cycling for more than 1000 cycles. These results demonstrate the great promise of hetero-symbiotic biphasic active materials as high-performance cathodes for sodium-ion batteries.