Key Takeaways & Executive Findings
- •• Fe–Mn dual-doping modulates the electronic structure of Na3V2(PO4)2F3, inducing a direct-to-indirect bandgap transition that enhances carrier concentration and lifetime. • The FM-NVPF cathode achieves a high capacity of 126.6 mAh g⁻1 at 0.1 C and exceptional rate capability of 67.6 mAh g⁻1 at 50 C, with a charge time of only 1.2 minutes. • Full cells with FM-NVPF cathode and hard carbon anode deliver a high energy density of ~175 Wh kg⁻1 (cathode+anode mass) and excellent cycling stability. • This work provides a cost-effective strategy using affordable Fe and Mn dopants to improve phase purity and electrochemical performance of NVPF cathodes for advanced sodium-ion batteries.
Abstract
Sodium superionic conductor (NASICON)-type materials are promising cathodes for sodium-ion batteries due to their stable multi-channel frameworks and exceptional ionic conductivity. Among them, Na3V2(PO4)2F3 (NVPF) has attracted significant attention. However, the low electronic conductivity and phase impurities limit its sodium storage capability. Herein, we present a Fe and Mn dual-doped NVPF (FM-NVPF) cathode with improved phase purity, electronic conductivity, and electrochemical activities. Detailed ex-situ analyses and density functional theory calculations reveal that Fe and Mn dopants induce defect energy levels and modulate the electronic structure, resulting in a direct-to-indirect bandgap transition in NVPF, which in turn increases carrier concentration and lifetime, accelerates ionic/electronic transport, and improves structural stability. As a result, the FM-NVPF cathode delivers a high capacity of 126.6 mAh g⁻1 at 0.1 C (1 C = 128 mAh g⁻1) and outstanding high-rate capability of 67.6 mAh g⁻1 at 50 C, corresponding to 1.2 min per charge. Furthermore, Na ion full cells assembled with the FM-NVPF cathodes and hard carbon anodes exhibit a high energy density of about 175 Wh kg⁻1 cathode+anode mass and appealing cyclic stability. This work provides an efficient strategy for developing high-purity and high-performance NVPF cathode materials for advanced sodium-ion batteries.
1. Introduction
With the increasing demand for sustainable energy storage solutions, sodium-ion batteries (SIBs) have emerged as promising alternatives to lithium-ion batteries due to the abundant and low-cost availability of sodium resources [1–3]. However, the development of SIBs remains challenged by the sluggish kinetics and limited cycling stability of available cathode materials [4, 5]. Cathode materials for SIBs mainly include layered transition metal oxides, polyanionic compounds, and Prussian blue analogs, among which polyanionic structures with three-dimensional ion conduction channels and rigid frameworks are specifically promising for long-life SIBs [6].
Na3V2(PO4)2F3 (NVPF) emerges as a particularly promising candidate in this category, featuring a three-dimensional framework of corner-sharing VO6 octahedra and PO4 tetrahedra that form interconnected channels for rapid Na+ diffusion [7]. While NVPF exhibits attractive characteristics including a high theoretical capacity (128 mAh g⁻1), appropriate redox potentials, and robust structural stability, its practical implementation is hindered by poor electronic conductivity and phase impurity formation during synthesis, which collectively degrade rate capability and cycling durability [8, 9].
To mitigate these obstacles, strategies such as carbon coating, nanosizing and transition metal doping have been employed to enhance the electronic conductivities, reduce the ion diffusion length, and regulate the electronic structures [10–12]. Among them, transition metal doping at the vanadium redox center serves multiple functions, including improving electronic conductivity, reducing Na ion diffusion energy barriers, and strengthening structural stability [13]. Affordable iron (Fe) and manganese (Mn) have attracted attention as dopants in NVPF cathode materials. For example, Park et al. prepared Fe-doped NVPF/Na3V2(PO4)2 (NVP) with rational Fe contents, which showed optimal performance of 118 mA h g−1 at 0.5 C under Fe-doping content of 8.9 at% [14]. Li et al. investigated the impact of Fe-dopant stoichiometry on the volcanic electronic conductivity and electron activation energy for Na3V2-2xFe2x(PO4)2F3, revealing a transition in the electron charge transfer process converts from V to Fe ions at x = 0.03 [15]. Zhang et al. utilized the polyol-assisted hydrothermal synthesis and chemical vapor deposition method to prepare Na3V1.95Mn0.05(PO4)2F3@C hollow microspheres, where the Mn2+ doping and carbon coating improved the electrical conductivity and Na ion diffusion coefficient [16]. Despite the successful integration of Fe or Mn doping individually, the synergistic effects of dual-doping on the electronic structure and electrochemical performance of NVPF remain underexplored. In this work, we present a Fe and Mn dual-doped NVPF (FM-NVPF) cathode that exhibits improved phase purity, electronic conductivity, and electrochemical activity. Our combined experimental and computational analyses reveal that Fe and Mn co-doping induces defect energy levels and modulates the electronic structure, leading to a direct-to-indirect bandgap transition. This transition enhances carrier concentration and lifetime, accelerates ionic/electronic transport, and improves structural stability. Consequently, the FM-NVPF cathode delivers exceptional rate capability and cycling stability, demonstrating a promising strategy for developing high-performance NVPF cathodes for advanced sodium-ion batteries.
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Jien Li, Shuang Luo, Renjie Li, Yingkai Hua, Linlong Lyu, Xiangjun Pu, Jun Fan, Zheng-Long Xu (2026). Iron–Manganese Dual-Doping Tailors the Electronic Structure of Na3V2(PO4)2F3 for High-Performance Sodium-Ion Batteries. Nano-Micro Letters. https://doi.org/10.1007/s40820-025-01881-3
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Frequently Asked Questions
What is the main challenge addressed in this paper?
The paper addresses the low electronic conductivity and phase impurities of Na3V2(PO4)2F3 (NVPF) cathode materials, which limit their sodium storage capability in sodium-ion batteries.
How does Fe-Mn dual-doping improve the performance of NVPF?
Fe-Mn dual-doping induces defect energy levels and modulates the electronic structure, causing a direct-to-indirect bandgap transition. This increases carrier concentration and lifetime, accelerates ionic/electronic transport, and improves structural stability, leading to enhanced rate capability and cycling stability.
What are the key electrochemical performance metrics of the FM-NVPF cathode?
The FM-NVPF cathode delivers a high capacity of 126.6 mAh g⁻1 at 0.1 C and outstanding high-rate capability of 67.6 mAh g⁻1 at 50 C, corresponding to a charge time of only 1.2 minutes.
What is the significance of the full cell performance?
Full cells assembled with FM-NVPF cathodes and hard carbon anodes exhibit a high energy density of about 175 Wh kg⁻1 (based on cathode+anode mass) and appealing cyclic stability, demonstrating practical applicability.
What methods were used to analyze the electronic structure changes?
The study employed detailed ex-situ analyses and density functional theory (DFT) calculations to reveal the modulation of electronic structure and the direct-to-indirect bandgap transition induced by Fe-Mn dual-doping.
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