Mechanistic Insights into Enhanced Capacity and Pure-Phase Formation in Fe-Based Mixed Phosphate Cathodes

High Resolution Image Download MS PowerPoint Slide Fe-based mixed phosphate Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 is a promising sodium-ion battery cathode due to its structural stability and cost-effectiveness, yet its capacity is limited by impurity phases and insufficient Fe redox activity. We introduce an electroactive coefficient (η = C/I ), where C is the number of redox couples and I is the number of transferred ions per formula unit, as a design metric for high-capacity cathodes. Analysis reveals that Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 has a suboptimal η (0.72), prompting a V-doping strategy to enhance multielectron transfer, raising η to 0.85. V doping also triggers a high-spin-to-low-spin transition in Fe 2+, shortening Fe–O bonds and increasing the Fe-vacancy formation energy, thus suppressing impurities. The optimized Na 3.6 Fe 2.6 V 0.4 (PO 4 ) 2 P 2 O 7 achieves a record capacity of 124.6 mAh g –1 at 0.1 C. This work elucidates phase-pure cathode formation and establishes a universal design principle for high-capacity electrodes.