Breaking Rate‐Limit of Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 by Quasi‐ALD Derived Ultra‐Low Carbon Coating in High‐Rate Sodium‐Ion Batteries

ABSTRACT Carbon coating is a key strategy to enhance electron transport for polyanionic cathode materials. However, current carbon‐coating methods face challenges in simultaneously achieving ultrathin, continuous, and highly conductive coatings with low carbon content, to enable efficient coupled ion–electron transport. As for Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 (NFPP), it still suffers from the above issues, limiting its electrochemical performance and industrial application. Commercial atomic layer deposition (ALD) technology is highly precise but unsuitable for carbon coatings and requires costly specialized equipment. Here, a precursor‐induced quasi‐ALD carbon deposition strategy is proposed to fabricate high‐power, stable NFPP@C‐FAC cathodes, using molecular‐level coupling of Fe and C sources. The obtained carbon matrix exhibits an ultrathin, uniform coating with an ultra‐low carbon content of 1.5% and few defects. These properties increase the electronic conductivity and the interfacial Na + diffusion kinetics, thereby improving the coupled ion–electron transport, as confirmed by DFT calculations. NFPP@C‐FAC demonstrates excellent rate performance (58 mAh g −1 at 200 C) and exceptional cycling stability, with 83.2% capacity retention after 20 000 cycles at 20 C. Moreover, NFPP@C‐FAC exhibits superior performance over a wide temperature range, which shows 90% capacity retention at −20°C compared to room temperature, with a single‐phase solid‐solution reaction mechanism confirmed by in situ XRD.