The global transition towards sustainable energy systems has intensified the search for efficient, cost-effective, and scalable energy storage solutions. While lithium-ion batteries (LIBs) have dominated this landscape, concerns regarding lithium resource scarcity and cost have propelled significant research into viable alternatives. Among these, sodium-ion battery technology stands out due to the natural abundance of sodium, lower cost, and electrochemical principles analogous to LIBs. The performance of a sodium-ion battery is critically dependent on its electrode materials, particularly the cathode, which dictates capacity, voltage, and cycle life. Polyanionic compounds have emerged as promising cathodes due to their structural stability, high operating voltage, and minimal volume change during cycling. Specifically, Na4Fe3(PO4)2P2O7 (NFPP) offers an attractive combination of a 3D open framework for facile Na+ diffusion, a respectable theoretical capacity (~129 mAh g-1), and the use of earth-abundant iron. However, its widespread adoption in sodium-ion battery applications is hindered by intrinsically low electronic and ionic conductivity.
To address these limitations, we devised a dual-modification strategy. Our approach combines cationic doping with conductive carbon coating to synergistically enhance both bulk and surface properties. We selected Mn2+ as the dopant due to its similar ionic radius to Fe2+, which facilitates lattice substitution, and its potential to act as a structural pillar. Simultaneously, integrating reduced graphene oxide (rGO) creates a pervasive conductive network that improves electron transfer at the particle surface and buffers volume changes. Herein, we report the synthesis, detailed characterization, and superior electrochemical performance of Mn2+-doped, rGO-coated Na4Fe3-xMnx(PO4)2P2O7 composites (Mnx-NFPP/rGO). We systematically investigate the influence of the Mn doping level (x) on the crystal structure, morphology, and ultimately, the sodium-ion battery performance. Furthermore, density functional theory (DFT) calculations provide fundamental insights into how Mn doping alters the electronic structure, thereby validating the observed enhancements in conductivity and kinetics.
We employed a facile sol-gel method followed by controlled thermal treatment to synthesize the Mnx-NFPP/rGO composites. Stoichiometric amounts of FeSO4·7H2O, MnSO4·H2O, NH4H2PO4, and CH3COONa were dissolved in deionized water with citric acid as a chelating agent. Aqueous graphene oxide (GO) dispersion was then introduced, and the mixture was stirred vigorously to ensure homogeneous mixing. The sol was converted to a gel by evaporation and subsequently dried to obtain a precursor. This precursor was first calcined in an inert atmosphere at an intermediate temperature to initiate crystallization and then sintered at an optimized higher temperature to obtain the final crystalline product, where GO is thermally reduced to rGO. Samples with nominal x = 0, 0.15, 0.30, and 0.45 were prepared, labeled as NFPP/rGO, Mn0.15-NFPP/rGO, Mn0.30-NFPP/rGO, and Mn0.45-NFPP/rGO, respectively. The composite architecture is designed to facilitate rapid ion and electron transport, which is crucial for high-power sodium-ion battery applications.
The phase purity and crystal structure of all synthesized materials were confirmed by X-ray diffraction (XRD). All diffraction patterns could be indexed to a pure orthorhombic phase with the Pn21a space group, matching the standard NFPP structure. No impurity phases were detected, indicating successful incorporation of Mn into the Fe sites without altering the fundamental polyanionic framework. A closer examination revealed a systematic shift of the major diffraction peaks (e.g., (200), (011), (210)) to slightly lower angles with increasing Mn content. This shift is direct evidence of lattice expansion, as the larger Mn2+ ion (ionic radius ~0.067 nm for high-spin) substitutes for the smaller Fe2+ ion (~0.061 nm for high-spin). Rietveld refinement was performed to quantify these changes, and the extracted lattice parameters are summarized in Table 1.
| Sample | a (Å) | b (Å) | c (Å) | Cell Volume (Å3) |
|---|---|---|---|---|
| NFPP/rGO | 17.968 | 6.438 | 10.382 | 1200.8 |
| Mn0.15-NFPP/rGO | 18.107 | 6.440 | 10.358 | 1207.5 |
| Mn0.30-NFPP/rGO | 18.119 | 6.443 | 10.369 | 1210.2 |
| Mn0.45-NFPP/rGO | 18.143 | 6.457 | 10.370 | 1214.6 |
The monotonic increase in cell volume confirms the successful doping and lattice expansion. This expansion is strategically beneficial for a sodium-ion battery cathode, as it can widen the ionic diffusion channels, effectively reducing the energy barrier for Na+ (de)insertion. The relationship between the diffusion coefficient (D) and the effective channel radius (rchannel) can be conceptually framed by considering the steric effects, often related to the Haven ratio and ionic mobility:
$$ D_{Na^+} \propto \frac{1}{\lambda} \cdot \frac{k_B T}{6\pi \eta r_{ion}} \cdot f(r_{channel}/r_{ion}) $$
where \(k_B\) is Boltzmann’s constant, \(T\) is temperature, \(\eta\) is viscosity, \(r_{ion}\) is the radius of the Na+ ion, \(\lambda\) is the jump distance, and \(f\) is a function that increases as the channel radius \(r_{channel}\) increases relative to \(r_{ion}\). Therefore, lattice expansion from Mn doping directly contributes to enhanced ionic kinetics, a key factor for rate capability in a sodium-ion battery.
Morphological analysis via scanning electron microscopy (SEM) revealed that all composites exhibit a similar three-dimensional porous architecture, where NFPP-based nanoparticles are uniformly embedded within and interconnected by the rGO sheets. The rGO forms a continuous, wrinkled conductive matrix that wraps the active material particles, ensuring good electrical contact. This intertwined structure provides numerous pathways for electron conduction and allows electrolyte penetration, facilitating rapid charge transfer reactions. High-resolution transmission electron microscopy (HRTEM) of the optimal Mn0.30-NFPP/rGO sample showed clear lattice fringes corresponding to the (020) plane of the orthorhombic NFPP, with an interplanar spacing of approximately 0.321 nm, confirming high crystallinity. An amorphous carbon layer (~3-5 nm thick) was observed at the interface, attributable to the rGO coating and residual carbon from the citrate. Energy-dispersive X-ray spectroscopy (EDS) elemental mapping confirmed the homogeneous distribution of Na, Fe, Mn, P, O, and C throughout the composite, verifying the successful and uniform integration of Mn and rGO. X-ray photoelectron spectroscopy (XPS) further validated the chemical states, showing Fe 2p peaks corresponding to Fe2+ and Mn 2p peaks indicative of Mn2+, confirming the intended dopant oxidation state was preserved during synthesis.
The electrochemical performance of the Mnx-NFPP/rGO composites as cathodes for sodium-ion battery was evaluated in half-cells against sodium metal. Figure 1a shows the initial galvanostatic charge-discharge profiles at a low current rate of 0.05 C. All profiles exhibit characteristic flat plateaus around 3.0 V vs. Na+/Na, corresponding to the Fe2+/Fe3+ redox couple. The initial discharge capacity shows a clear dependence on Mn content, with Mn0.30-NFPP/rGO delivering the highest value of 131.2 mAh g-1, closely approaching the theoretical limit. This surpasses the capacity of the undoped NFPP/rGO (120.1 mAh g-1). The enhancement can be attributed to the improved Na+ diffusion kinetics and electronic conductivity due to Mn doping. However, excessive doping (x=0.45) led to a slight capacity reduction to 128.0 mAh g-1, possibly due to the dilution of the electrochemically active Fe sites or minor structural distortions.
The rate capability, a critical metric for high-power sodium-ion battery applications, was systematically tested. As presented in Table 2 and the corresponding rate performance plot, Mn0.30-NFPP/rGO consistently outperformed other samples across various current densities from 0.1 C to 2 C.
| Sample | Discharge Capacity @ 0.05C (mAh g-1) | Discharge Capacity @ 2C (mAh g-1) | Capacity Retention @ 1C after 100 cycles | Charge Transfer Resistance, Rct (Ω) |
|---|---|---|---|---|
| NFPP/rGO | 120.1 | 78.5 | 87% | 694.5 |
| Mn0.15-NFPP/rGO | 125.3 | 85.0 | 89% | 530.1 |
| Mn0.30-NFPP/rGO | 131.2 | 91.9 | 94% | 353.1 |
| Mn0.45-NFPP/rGO | 128.0 | 90.8 | 91% | 390.1 |
The superior rate performance of Mn0.30-NFPP/rGO is directly linked to its enhanced kinetics. The relationship between the achievable capacity at a high rate (Crate) and the material’s properties can be expressed by considering the polarization:
$$ \eta_{total} = \eta_{ohm} + \eta_{ct} + \eta_{diff} = iR_{\Omega} + \frac{RT}{\alpha nF} \ln(\frac{i}{i_0}) + \frac{RT}{nF} \text{arcsinh}(\frac{i}{2i_L}) $$
where \(i\) is current density, \(R_{\Omega}\) is ohmic resistance, \(i_0\) is exchange current density (inversely related to Rct), and \(i_L\) is the limiting diffusion current. Mn doping reduces both \(R_{\Omega}\) (by improving electronic conductivity) and \(R_{ct}\) (by facilitating charge transfer), while lattice expansion increases \(i_L\) by boosting the solid-state diffusion coefficient \(D_{Na^+}\). This multi-faceted reduction in overpotential (\(\eta_{total}\)) allows the Mn0.30-NFPP/rGO cathode to maintain higher capacity under demanding current loads. When the current rate was returned to 0.1 C, the capacity nearly fully recovered, demonstrating excellent electrochemical reversibility and structural resilience.
Long-term cycle stability is another cornerstone for practical sodium-ion battery deployment. As shown in Table 2, Mn0.30-NFPP/rGO exhibited the best capacity retention of 94% after 100 cycles at 1 C, with a retained discharge capacity of 101.0 mAh g-1. The undoped sample retained only 87%. The remarkably stable cycling stems from the synergistic effects of the modifications: the robust rGO network maintains electrical integrity and accommodates mechanical strain, while the Mn2+ dopant acts as a structural stabilizer within the crystal lattice, mitigating detrimental phase transitions or lattice collapse during repetitive Na+ extraction/insertion. Electrochemical impedance spectroscopy (EIS) measurements performed on cycled cells provided quantitative support. The Nyquist plots comprised a semicircle in the high-medium frequency region (associated with charge transfer resistance, Rct) and a sloping line in the low-frequency region (related to Na+ solid-state diffusion, Ws). The fitted Rct values, listed in Table 2, show a minimum for Mn0.30-NFPP/rGO (353.1 Ω), significantly lower than that of NFPP/rGO (694.5 Ω). This confirms that Mn doping effectively lowers the interfacial charge-transfer barrier, accelerating the reaction kinetics. Furthermore, the steeper Warburg slope for the doped samples indicates a higher Na+ diffusion coefficient, consistent with the expanded lattice channels.
To gain fundamental insight into the electronic structure modulation induced by Mn doping, we performed density functional theory (DFT) calculations on the pristine NFPP and Mn0.30-NFPP (modeled by substituting one Fe in the unit cell). The calculated band structures and projected density of states (PDOS) revealed crucial differences. Both materials are indirect band gap semiconductors. The calculated band gap for pristine NFPP was 3.569 eV. Upon Mn doping, the band gap reduced to 3.128 eV for Mn0.30-NFPP. A narrower band gap signifies that electrons in the valence band can be more easily excited into the conduction band, which intrinsically improves electronic conductivity. Analysis of the PDOS showed that for both structures, the valence band maximum (VBM) is primarily composed of O 2p and P 3p states, while the conduction band minimum (CBM) is dominated by Fe 3d states. In the Mn-doped system, the Mn 3d states contribute significantly near the Fermi level, introducing additional electronic states that effectively reduce the band gap. The enhancement in intrinsic electronic conductivity (\(\sigma\)) can be qualitatively related to the band gap (\(E_g\)) via an Arrhenius-type relation for semiconductor carrier concentration:
$$ n \propto \exp\left(-\frac{E_g}{2k_B T}\right) $$
$$ \sigma = n e \mu $$
where \(n\) is the charge carrier concentration, \(e\) is the elementary charge, and \(\mu\) is the carrier mobility. A reduction in \(E_g\) leads to an exponential increase in \(n\), thereby boosting \(\sigma\). This theoretical finding provides a first-principles explanation for the improved electrochemical kinetics and rate performance observed in our Mn-doped sodium-ion battery cathode. The synergistic effect can be summarized by a conceptual performance metric (P) for the cathode:
$$ P \approx \frac{Q_{theory} \cdot \sigma_{eff} \cdot D_{Na^+}}{R_{ct} + R_{\Omega}} $$
where \(Q_{theory}\) is the theoretical capacity, \(\sigma_{eff}\) is the effective conductivity (enhanced by rGO and lower \(E_g\)), and \(D_{Na^+}\) is the diffusion coefficient (enhanced by lattice expansion). Our dual-modification strategy maximizes this metric by positively influencing all terms in the numerator and minimizing those in the denominator.
In conclusion, we have successfully designed and synthesized a high-performance polyanionic cathode for sodium-ion battery through a rational dual-modification strategy. The incorporation of Mn2+ into the Na4Fe3(PO4)2P2O7 lattice induces beneficial lattice expansion, widening Na+ diffusion pathways and enhancing ionic transport. Concurrently, wrapping the active material particles with a conductive rGO network ensures efficient electron transfer and structural robustness. The optimal composition, Mn0.30-NFPP/rGO, delivers an exceptional combination of high specific capacity (131.2 mAh g-1 at 0.05 C), outstanding rate capability (91.9 mAh g-1 at 2 C), and remarkable cycling stability (94% capacity retention after 100 cycles at 1 C). DFT calculations corroborate that Mn doping effectively narrows the band gap of the material, elevating its intrinsic electronic conductivity. This work elucidates the structure-property-performance relationships in modified polyanionic compounds and demonstrates a potent materials engineering approach. The synergistic combination of bulk doping and nano-scale conductive coating presents a generalizable pathway for developing advanced, cost-effective electrode materials, accelerating the progress towards commercially viable and high-performance sodium-ion battery technology for large-scale energy storage.