Dual-Carbon-Coated Na2FeP2O7 Cathode Materials for Na-Ion Batteries with Superior High-Rate and Cycling Stability

The iron-based pyrophosphate Na₂FeP₂O₇ (NFP) is considered as one of the most promising cathodes for sodium-ion batteries (SIBs) due to its low-cost and superior structure stability, yet it usually suffers from poor intrinsic electronic conductivity. Herein, a two-step carbon-coating technique has been developed to synthesize high-performance NFP@C cathode materials by controlling the NFP particle size and the coating layer uniformity. The first step of in-situ carbon coating greatly restrains the excessive growth of NFP crystals with a shortened Na-ion diffusion path. Meantime, the extra secondary carbon-coating is adopted to repair some exposed areas, guaranteeing the full coverage of NFP particles for rapid electronic transfer. As a consequence, the as-obtained NFP@C cathode delivers a high discharge capacity of 95.2 mAh g⁻¹ at 0.1 C (theoretical value: 97 mAh g⁻¹) and with high-rate capability (75.2 mAh g⁻¹ at 5 C) within 2.0–4.0 V. A capacity retention of 95.3% can be achieved even after 500 cycles at 5 C in coin-type cells. Such superior electrochemical performances are expected to quickly promote the applications of NFP in SIBs.