Structural properties and electrochemical performance of the polyanionic Na₄Fe₃(PO₄)₂(P₂O₇)@C cathode at lower cutoff...

Developing cathode materials with superior framework stability, affordability, and eco-friendliness is a key focus of sodium-ion battery research. Herein, a Na₄Fe₃(PO₄)₂(P₂O₇) carbon-coated (NFPP@C) composite is synthesized through an adjustable calcination process using the sol-gel method to enhance cathode conductivity and electrochemical performance. The study offers new insights into the electrochemical behaviour of NFPP-based polyanion systems at low-potential storage sites, particularly at a lower cutoff potential of 0.5 V. The results demonstrate the ability of the NFPP@C cathode to operate within a lower cutoff potential window (0.5-3.5 V) while maintaining a significant specific capacity, delivering an initial specific capacity of 124 mAh g^-1 at 0.05 A g^-1 and 105 mAh g^-1 at 0.1 A g^-1 over 500 cycles, with the capacity increasing with cycling. However, achieving excellent specific capacity comes at the expense of battery health, i.e., the cathode material loses its significant structure, degrading the voltage plateau where the specific phase occurs during the discharge/charge cycle, increasing the polarization by an IR drop and plateau difference, and increasing the cathode electrolyte interface (CEI). To emphasize these results, the potential window of 1-3.5 V is studied to explore the impact of deep discharge. At the potential window of 1-3.5 V, the cathode demonstrates excellent capacity stability with a coulombic efficiency (CE) of 99%, delivering a specific capacity of 70 mAh g^-1 over 300 cycles. Moreover, the cathode maintains its phase transitions, exhibiting uniform charge and discharge plateaus, low polarization, and low total resistance. Eventually, this study lays the groundwork for further research on the potential window of the NFPP@C cathode for sodium-ion batteries and the development of a mechanistic strategy.