Unveiling the Key Factors: Mn/Fe Ratio, Distribution Homogeneity, and Electronic Conductivity of Na4MnxFe3-x(PO4)2P2O7 for Practical Sodium-Ion Batteries

Abstract

High Mn content Na₄Mn_xFe_3-x(PO₄)₂P₂O₇ materials are attractive cathodes for sodium-ion batteries with a higher operating voltage than that of Na₄Fe₃(PO₄)₂P₂O₇. However, the poor intrinsic electronic conductivity and the Jahn–Teller distortion caused by Mn³⁺ still make it difficult to obtain satisfactory Na₄Mn_xFe_3-x(PO₄)₂P₂O₇ up to now. In this work, Na₄Mn_xFe_3-x(PO₄)₂P₂O₇/C-CNT is successfully synthesized with a uniformly distributed Mn/Fe structure using Mn/Fe oxalate coprecipitates as precursors. The Na₄Mn_1.7Fe_1.3(PO₄)₂P₂O₇/C-CNT sample with optimal Mn/Fe ratio and enhanced electronic conductivity exhibits a high mid-discharge voltage of 3.70 V versus Na/Na⁺ with a reversible capacity of 111 mAh g⁻¹ at 0.05C, and also excellent rate capability (89.4 mAh g⁻¹ at 20C) and superior cycling stability (90.4% capacity retention after 1000 cycles at 0.5C). An appropriate Mn/Fe ratio and uniform Mn/Fe distribution in the Na₄Mn_1.7Fe_1.3(PO₄)₂P₂O₇ structure are critical to suppress the Jahn–Teller distortion of Mn³⁺. DFT calculation indicates the necessity of improving its electronic conductivity compared to that of conventional Na₄Fe₃(PO₄)₂P₂O₇. This work opens up valuable insights and strategies to design novel Na₄Mn_xFe_3-x(PO₄)₂P₂O₇ cathodes for practical sodium-ion batteries.