Mechanistic Insights into Enhanced Capacity and Pure-Phase Formation in Fe-Based Mixed Phosphate Cathodes
Wande Song, Nan Chen, Jinpeng Wang, Weibo Hua, Bohan Zhang, Shuoqing Zhao, Xing Ming, Zexiang Shen, Gang Chen, Fei Du
Abstract
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.