Fe-Doped Non-stoichiometric Na3.4Co2.4(PO4)1.4P2O7 Cathode for High-Energy-Density Sodium-Ion Batteries.

Na4Co3(PO4)2P2O7 is a representative mixed polyanionic cathode with a high operating voltage and high energy density. However, insufficient cycling stability is the core bottleneck constraining the widespread application of sodium-ion batteries (SIBs). Herein, we propose a synergistic strategy that combines non-stoichiometry with Fe2+ doping to construct Na3.4Co2.4-xFex(PO4)1.4P2O7 (NCFPP-x), which not only enhances phase purity but also improves material stability. Through ex situ electrochemical impedance spectroscopy-distribution of relaxation times and galvanostatic intermittent titration technique analyses, it is demonstrated that Fe doping leads to the reduced charge transfer impedance and enhanced Na+ diffusion coefficient. The in situ XRD characterization results demonstrate a highly reversible incomplete solid-solution reaction mechanism during the Na+ intercalation and deintercalation processes. The NCFPP-1.2||Hard carbon (HC) full cell assembled with an HC anode delivers an energy density of 325 Wh kg-1, retaining 80.0% after 200 cycles. The present work offers important theoretical basis and experimental support for the advancement of high-voltage polyanion-type cathode materials.