Facile fabrication of the hybrid of amorphous FePO4.2H2O and GO toward high performance sodium-ion batteries

Iron phosphate (FePO4) with moderate working voltage is an appealing cathode material for room temperature rechargeable sodium-ion batteries (SIBs), while the low electronic conductivity and poor cycle performance remarkably impede its possible energy storage applications. In this work, we report a simple route to fabricate graphene oxide (GO) hybridized amorphous FePO4.2H2O composite for SIBs. By directly mixing FeCl3.6H2O and GO (mass controlled by x wt% of the mass of FeCl3.6H2O) in an agate mortar with pestle, Fe3+ ions could be bound by the oxygen-containing functionalities on the surface of GO nanosheets. After adding NH4H2PO4 powder to the above mixture, a direct room temperature solid state reaction between the GO-confined Fe3+ ions and PO43-polyanions results in an in-situ hybridization of amorphous FePO4.2H2O and GO, i.e. the target electro-active material (denoted as FePO4@GO-x) is attained. In-depth mechanism for the solid state reaction is demonstrated by FTIR and XPS techniques. The optimized hybrid of amorphous FePO4.2H2O and GO (FePO4@GO-15 composite) exhibits the best sodium storage performance. A high reversible charge capacity of 129.0 mAh g-1 is delivered at a current density of 100 mA g-1 for the first cycle, which is around 90% of the theoretical capacity (144 mAh g-1) of FePO4.2H2O. The composite also exhibits outstanding rate performance, superior reversibility, and satisfactory long-lasting cycle stability. When cycled at a high current density (800 mA g-1 at 31 degrees C), the composite cathode still delivers a charge capacity of 51 mAh g-1 over 1000 cycles, cor-responding to 67% of the first charge capacity. The results presented clearly indicate that the hybrid of amor-phous FePO4.2H2O and GO is a promising cathode material for SIBs.