Facile synthesis of hierarchical Na2Fe(SO4)2@rGO/C as high-voltage cathode for energy density-enhanced sodium-ion batteries

Fe-based sulfates are ideal cathode candidates for sodium-ion batteries (SIBs) owing to their high operating voltage and low cost but suffer from the nature of poor power performance. Herein, a hierarchical porous Na2Fe(SO4)2@reduced graphene oxide/carbon dot (Na2Fe(SO4)2@rGO/C) with low carbon content (4.12 wt%) was synthesized via a facile homogeneous strategy benefiting for engineering application, which delivers excellent sodium storage performance (high voltage plateau of 3.75 V, 85 mAh g−1 and 330 Wh kg−1 at 0.05 C; 5805 W kg−1 at 10 C) and high Na+ diffusion coefficient (1.19 × 10−12 cm2 s−1). Moreover, the midpoint voltage of assembled full cell could reach 3.0 V. The electron transfer and reaction kinetics are effectively boosted since the nanoscale Na2Fe(SO4)2 is supported by a robust cross-linked carbon matrix with rGO sheets and carbon dots. The slight rGO sheets sufficiently enhance the electron transfer like a current collecter and restrain the aggregation, as well as ensure smooth ion channels. Meanwhile, the carbon dots in the whole space connect with Na2Fe(SO4)2 and help rGO to promote the conductivity of the electrode. Ex-situ X-ray powder diffraction and X-ray photoelectron spectrometry analysis confirm the high reversibility of this sodiation/desodiation process.