Dual-strategy of carbon-coating and nanoengineering enables reversible and durable Na storage in an iron-based pyrophosphate cathode

Abstract

Iron-based polyanionic compounds exhibit promising potential as commercial cathode materials for sodium-ion batteries (SIBs) due to their abundant resources, low cost, environmental friendliness, and reliability. However, they still encounter challenges such as limited energy density, inadequate ionic/electronic conductivity, and sluggish Na⁺ diffusion kinetics. Herein, the Na storage performance of an iron-based pyrophosphate cathode is enhanced through a dual-strategy involving in situ carbon-coating and nanoengineering. Uniform carbon-coated Na_3.12Fe_2.44(P₂O₇)₂/C (NFPO/C) nanoflakes are synthesized via solid-state reaction in a molten surfactant–paraffin medium. Based on the synergistic effects of surface coating and nanotechnology, NFPO/C nanoflakes demonstrate excellent kinetic performance and remarkable structural reversibility during Na storage. Consequently, the NFPO/C cathode exhibits a high reversible capacity of 101.6 mA h g⁻¹ at 1C and demonstrates exceptional cycling stability, retaining 79% of its capacity after 3700 cycles at 2C. Furthermore, the full cell assembled using an NFPO/C cathode and a hard carbon (HC) anode demonstrates a high reversible capacity of 92 mA h g⁻¹ at 1C and exhibits excellent cycling stability with 72% capacity retention after 200 cycles at 1C. These findings suggest that the iron-based pyrophosphate cathode holds great promise as a potential candidate for commercial SIBs.