Heteroatom Doping Strategy for Enhanced Sodium-Ion Storage in Na₂Fe_1.5Mn_1.5(PO₄)₃

Sodium-ion batteries (SIBs) are promising and cost-effective substitutes for lithium-ion batteries for large-scale energy storage. Hence, exploring novel anode materials is crucial to developing sustainable SIBs. Herein, a nitrogen and sulfur co-doped carbon layer wrapped alluaudite Na₂Fe_1.5Mn_1.5(PO₄)₃ (NFMP@SNC) with uniform 3D urchin-like morphology is successfully synthesized via a simple hydrothermal technique. For the first time, this study examines their electrochemical properties as an anode for SIBs. The N, S-doped carbon layer forms a conductive network that enhances electron transport, facilitates Na⁺ diffusion, and prevents particle aggregation and side reactions. As a result, NFMP@SNC displays an irreversible capacity of 774.52 mAh g^-1 and a reversible capacity of 253.40 mAh g^-1 at 0.05C, retaining 61.2% of its theoretical capacity (414 mAh g^-1). Furthermore, it shows an excellent rate capability of 71.76% at 0.1C (25 cycles) and retention of 48.49% at 0.2C (100 cycles). Additionally, density functional theory (DFT) calculations are conducted to evaluate the electronic band structure, density of states, charge density distribution, and Na⁺ diffusion energy barriers of pristine NFMP, providing fundamental insights into its electrochemical behavior. With a low average voltage of ≈0.7 V, NFMP@SNC emerges as a promising intercalation-type anode material enabled by 3D architecture and N,S co-doping for high-performance SIBs.