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

Fe-based mixed phosphate Na₄Fe₃(PO₄)₂P₂O₇ is a promising sodium-ion battery cathode due to its structural stability and cost-effectiveness, <i>yet</i> its capacity is limited by impurity phases and insufficient Fe redox activity. We introduce an electroactive coefficient (η = <i>C/I</i>), where <i>C</i> is the number of redox couples and <i>I</i> is the number of transferred ions <i>per</i> formula unit, as a design metric for high-capacity cathodes. Analysis reveals that Na₄Fe₃(PO₄)₂P₂O₇ 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 Fe²⁺, shortening Fe-O bonds and increasing the Fe-vacancy formation energy, thus suppressing impurities. The optimized Na_3.6Fe_2.6V_0.4(PO₄)₂P₂O₇ 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.