Understanding the Role of Mn Substitution for Boosting High‐Voltage Na₄Fe_3‐xMn_x(PO₄)₂P₂O₇ Cathode in Sodium‐Ion Batteries

Na₄Fe₃(PO₄)₂P₂O₇ is regarded as the most promising polyanionic cathode for sodium-ion batteries (SIBs) due to its superior structural stability, cost-effectiveness, and environmental benignity. However, the low operating voltage inevitably weakens its competitiveness in energy density. Previous works have tried to enhance its operating voltage by Mn doping, which draws on the design idea of LiFe_xMn_1-xPO₄ cathode for lithium-ion batteries, but with little success. In this context, uncovering the role of Mn substitution in Na₄Fe_3-xMn_x(PO₄)₂P₂O₇ (NFM_xPP) cathode is urgently needed. This work discloses the effect of Mn contents on the structure, sodium storage property, and reaction mechanism of NFM_xPP cathode for the first time. Introducing a moderate amount of Mn (0.6 ≤ x ≤ 1.2) into NFM_xPP can weaken the Fe-O bonding interaction, thus leading to the full utilization of Mn³⁺/Mn²⁺ redox couple. As the representative, NFM_1.2PP cathode exhibited a high operating voltage of ≈3.3 V with a reversible capacity of 109.2 mAh g^-1. Note that a Hard carbon||NFM_1.2PP full battery manifests considerably high-capacity retention of 92.3% over 1600 cycles. It is believed that an understanding of the role of Mn substitution in this work will promote the practical application of high voltage NFM_xPP cathodes for SIBs.