Fe-substituted Na_3.5V_1.5Mn_0.5(PO₄)₃ NaSICON cathode with multi-electron reactions and improved energy output

Abstract

Sodium-ion batteries have emerged as a promising alternative to conventional lithium-ion batteries due to the abundance and low cost of sodium resources. NaSICON-based materials represent a highly competitive class of cathodes for sodium-ion batteries. Engineering NaSICON-type cathodes through rational design is a compelling approach to enhancing the overall energy density of sodium storage systems. In this study, the known Na_3.5V_1.5Mn_0.5(PO₄)₃ phase was strategically modified by partially substituting Mn with Fe using a Pechini route, followed by an ex situ carbon coating, leading to the formation of a novel ternary NaSICON-type Na_3.5V_1.5Mn_0.25Fe_0.25(PO₄)₃/C (NVMFP/C) cathode with enhanced properties. Structural and morphological analyses confirmed the high crystallinity and phase purity of the designed material (NVMFP/C). Electrochemical evaluation versus Na⁺/Na demonstrated a reversible capacity of 167 mA h g⁻¹ (∼0.30 mA h cm⁻²) at 0.1C, outstanding rate capability, and a high energy density of 412 Wh kg⁻¹ at 1C. Furthermore, the material exhibited remarkable cycling stability, with 74% capacity retention after 1400 cycles at 10C within the 1.5–4.3 V potential window. This promising electrochemical performance is attributed to the fast Na⁺ diffusion kinetics, as confirmed by the galvanostatic intermittent titration technique (GITT), and the structural stabilization, resulting from this compositional engineering. Moreover, the synergy between the Pechini route, ex situ carbon coating, and Fe incorporation activates multiple redox reactions and enables the designed cathode to operate within an expanded potential range with significant capacitive contribution, providing a robust pathway toward high-energy-density sodium-ion batteries.

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1. Introduction

The growing demand for energy storage systems, driven by continuous technological progress, has substantially increased the consumption of critical raw materials required for their fabrication.^1–3 Among these systems, lithium-ion batteries (LIBs) currently dominate the market. However, the scarcity of lithium and its uneven geographic distribution pose challenges to long-term sustainability, highlighting the urgent need for alternative materials.^3–5 In this context, sodium has attracted considerable attention due to its natural abundance and low cost, positioning sodium-ion batteries (SIBs) as a promising replacement for LIBs. Importantly, SIBs operate through a “rocking-chair” mechanism similar to that of LIBs, enabling comparable working principles while offering a more resource-sustainable pathway.^6,7

Since the appearance of SIBs, enormous amounts of effort have been deployed to develop new components for them, especially cathode materials, which are key determinants of the battery's total electrochemical capacity.^8,9 The employment of a high-voltage and large-capacity cathode material promotes the energy density and facilitates the practical application of SIBs.¹⁰ Cathode materials for SIBs fall mainly into four categories: organic compounds, Prussian blue analogs, transition metal oxides, and polyanionic compounds.¹¹ Among these, NaSICON-phosphate, which belongs to the polyanionic compounds, has received great attention due to its robust framework resulting from stable P–O covalent bonds that provide crucial structural stability and generate spacious 3D interstices.^12–16 While developing high-capacity cathode materials is essential, the overall performance and safety of sodium-ion batteries also depend on the electrolyte system. Recent advances in supramolecular gel electrolytes (SGEs) have shown that self-assembled network structures can significantly improve ion transport and mechanical stability.¹⁷ Combining such innovative electrolyte strategies with robust NaSICON-type frameworks could lead to the next generation of safe and high-energy-density storage devices.

In recent decades, the phosphate Na₃V₂(PO₄)₃ (NVP) has been extensively studied as a cathode for sodium-ion batteries due to its relatively high operating voltage (∼3.4 V) and remarkable structural stability. Nevertheless, the practical application of this compound is limited by various factors, including its unsatisfactory capacity and energy density, as well as the toxicity of vanadium.⁷ To address these limitations, various strategies have been employed, including the substitution of vanadium with low-cost and eco-friendly cations (e.g., Fe and Mn),^18,19 carbon coating²⁰ and nanosizing.²¹ In fact, metal cation doping is considered one of the most effective approaches to enhance the electrochemical performance of NVP.²²

Recently, Na_3+xV_2−xMn_x(PO₄)₃ (NVMP) has been extensively investigated as a cathode material for SIBs, especially the Na_3.5V_1.5Mn_0.5(PO₄)₃ NaSICON, driven by the low cost of Mn and its ability to enhance the operating voltage owing to the high voltage (3.6 V) of the Mn²⁺/Mn³⁺ couple compared to that of the V³⁺/V⁴⁺ couple (3.4 V).^10,23–27 Indeed, the substitution of V³⁺ with Mn²⁺ ion in NVP material enhances performance by increasing the charge carrier concentration and enlarging the lattice volume, which significantly facilitates the rapid diffusion of sodium ions.²⁸ Additionally, the synergistic effect of the V³⁺/V⁴⁺, V⁴⁺/V⁵⁺, and Mn²⁺/Mn³⁺ multi-electron reactions enables the appearance of three distinct voltage plateaus at 3.3, 3.6, and 3.9 V upon extending the cut-off voltage to 4.3 V.²⁹ Specifically, it has been shown that incorporating manganese at a content of up to x = 0.25 into NVMP triggers a beneficial V⁴⁺/V⁵⁺ redox reaction at 4.0 V and enhances the material's capacity, which ultimately improves the operating voltage and energy density of the material.¹⁰ However, due to the Jahn–Teller distortion inherent in manganese ions, Mn-rich cathode materials such as Na₄MnV(PO₄)₃ face significant challenges in terms of electrochemical stability.^19,30 Therefore, to further optimize the amount of manganese and enhance the electrochemical properties, the introduction of an additional inexpensive active element into NVMP is a promising approach. Indeed, bimetallic doping of NVP materials is considered an effective strategy for achieving multi-electron reactions and improving both electrical conductivity and structural stability.³¹ Previous research has demonstrated that the partial replacement of Mn²⁺ with Mg²⁺ and Al³⁺ in the Na₄MnV(PO₄)₃ lattice significantly improves the electrochemical behavior of the host material. These modified compounds displayed smoother voltage profiles, faster Na⁺ transport, improved rate capability, and remarkable cycling durability, retaining about 96% of their capacity after 100 cycles at 1C.³² More recently, Xusheng Zhang and co-workers reported the successful design of Na_3.3V_1.5Mn_0.3Al_0.2(PO₄)₃, derived from the parent Na_3.5V_1.5Mn_0.5(PO₄)₃ through partial substitution of Mn²⁺ with Al³⁺. The superior performance of this new phase was attributed to the suppression of Jahn–Teller distortion, achieved by tuning the local MnO₆ octahedral environment and strengthening Mn–O interactions.³³

Building upon these promising findings, a novel stoichiometry of V/Mn/Fe-based NaSICON Na_3.5V_1.5Mn_0.25Fe_0.25(PO₄)₃ (denoted as NVMFP) was synthesized in this work using a Pechini route. As demonstrated in our previous work,³⁴ the Pechini route effectively minimizes particle size, which can contribute to improving the electrochemical properties of the material. Additionally, a low-cost ex situ carbon coating was applied using sucrose to promote electron transfer within the NVMFP material, yielding NVMFP/C. Structural characterization confirmed the purity of the synthesized NVMFP/C powder, which crystallized in a trigonal system with the R3̄c symmetry. Electrochemical tests in a sodium half-cell configuration revealed that the NVMFP/C cathode delivers a discharge capacity of 167 mA h g⁻¹ at 0.1C over a 1.5–4.3 V potential window, corresponding to an areal capacity of ∼0.30 mA h cm⁻² based on an active mass loading of ∼2 mg cm⁻². This novel stoichiometry exhibits a competitive energy density compared to many reported V/Mn/Fe-based NaSICONs,^{27,33,35–38} making this material a strong candidate for a high-performance SIB cathodes.

2. Experimental procedure

2.1. Material preparation

The Na_3.5V_1.5Mn_0.25Fe_0.25(PO₄)₃ powder was synthesized using a Pechini approach, a wet chemical method that forms a stable polymeric complex to ensure uniform distribution of the metal cations and yields ultra-fine, chemically homogeneous powders. In this synthesis, sodium nitrate (NaNO₃, 99.5%, Solvachim), ammonium metavanadate (NH₄VO₃, 99%, Sigma-Aldrich), manganese acetate ((CH₃CO₂)₂Mn, 98%, Sigma-Aldrich), iron oxalate dihydrate (FeC₂O₄·2H₂O, 99%, Sigma-Aldrich), and ammonium dihydrogen phosphate (NH₄H₂PO₄, ≥99%, Sigma-Aldrich) were employed as precursors in a molar ratio of 3.5 : 1.5 : 0.25 : 0.25 : 3, respectively. Initially, NaNO₃, (CH₃CO₂)₂Mn, and FeC₂O₄·2H₂O were dissolved in 20 mL of distilled water with 4 mL of HNO₃ in a beaker immersed in a silicone oil bath under heating at 60 °C. NH₄VO₃ was then introduced, and the solution was stirred at 60 °C for 30 minutes. In parallel, citric acid (molar ratio 1 : 3 relative to metals) and NH₄H₂PO₄ were solubilized in 20 mL of distilled water and introduced into the mixture. A blend of ethylene glycol (99.5%, Panreac) and 10 mL of H₂O, with a 4 : 1 molar ratio of ethylene glycol to citric acid, was subsequently incorporated. The resulting solution was heated at 80 °C with continuous stirring until complete solvent evaporation occurred. The dried precursor was first heated in air at 190 °C for 4 hours, then ground and calcined at 750 °C for 10 hours under Ar atmosphere, using a ramp rate of 4 °C min⁻¹. To improve electronic conductivity, an ex situ carbon coating was applied. Sucrose (15 wt%) and the NVMFP powder (85 wt%) were mixed with 4 mL of acetone and ground thoroughly in an agate mortar for 20 minutes. The mixture was then pyrolyzed at 500 °C for 2 hours under an argon atmosphere with a heating rate of 5 °C min⁻¹ to yield the final carbon-coated product, Na_3.5V_1.5Mn_0.25Fe_0.25(PO₄)₃/C (NVMFP/C).

2.2. Material characterization

The crystal structures of NVMFP and NVMFP/C were examined using powder X-ray diffraction (XRD) on a Rigaku SmartLab diffractometer. Data were collected at ambient conditions using a 5° min⁻¹ scan rate, 0.02° step increments, and a 2θ interval of 10–100°. Rietveld refinement was conducted using FullProf software,³⁹ and schematic representations of the structures were generated with Diamond software.⁴⁰ Fourier-transform infrared (FT-IR) spectra were acquired on a Bruker spectrometer, covering the mid-infrared region from 400 to 4000 cm⁻¹. Raman spectra were collected using a Renishaw micro-Raman spectrometer over the 500–2300 cm⁻¹ frequency range. Thermal stability was investigated by thermogravimetric analysis (TGA) using a NETZSCH STA analyzer in air, with temperatures ranging from 25 to 600 °C and a heating rate of 10 °C min⁻¹. The morphological features and elemental distribution of the powders were examined using field-emission scanning electron microscopy (FEG-ESEM-FEI) equipped with energy-dispersive X-ray spectroscopy (EDS), along with transmission electron microscopy (TEM, Talos F200S).

2.3. Electrochemical tests

Cathode electrodes were fabricated from a slurry containing polyvinylidene fluoride (PVDF) binder, carbon black and active material in a mass ratio of 1 : 2 : 7, using N-methyl-2-pyrrolidone (NMP) as solvent. The resulting slurry was applied onto the Al foil substrate and dried for 4 hours at room temperature and at 120 °C in a conventional oven for another 4 hours. After drying, the slurry was shaped into discs with a diameter of 10 mm and dried at 120 °C for 12 hours under vacuum before being transferred to an Ar-filled glove box with H₂O and O₂ concentrations not exceeding 0.01 ppm. CR2032 coin cells were utilized to test the electrodes with an electrolyte consisting of 1 M NaClO₄ dissolved in a 50 : 50 (v/v) solvent mixture (ethylene carbonate (EC)/propylene carbonate (PC)) with 5 vol% of fluoroethylene carbonate (FEC). The cell configuration included sodium metal for the reference and counter electrodes and a GF/D glass fiber membrane as the separator. Electrochemical measurements, including galvanostatic cycling, cyclic voltammetry (CV), and the galvanostatic intermittent titration technique (GITT), were carried out on a Bio-Logic VMP-3 battery testing system. During the GITT experiments, the electrodes were subjected to charge/discharge steps of 30 minutes at a rate of 0.05C, each followed by a relaxation period of 2 hours. Electrochemical impedance spectroscopy (EIS) was performed by a Biologic SP-200 Potentiostat after the first charge and first discharge at C/20. The impedance data were fitted using ZView software. The electrodes had an active material mass loading of approximately 2 mg cm⁻². All electrochemical properties presented in this manuscript, including specific capacity and energy density, were normalized to the mass of the active material only. For ex situ XRD measurements, the cells were disassembled in an Ar-filled glove box and the electrodes were washed with dimethyl carbonate (DMC), dried, and covered with Kapton foil to prevent air exposure during data collection.

3. Results and discussion

3.1. Material characterization

NVMFP powder was successfully synthesized using a Pechini approach. The XRD patterns of NVMFP and NVMFP/C are identical (Fig. S1a). The Rietveld refinement of NVMFP/C is shown in Fig. 1a. The experimental XRD pattern displays no detectable impurities and correlates well with the calculated XRD pattern, with a satisfactory reliability factor, indicating that the structure of this phosphate is consistent with a trigonal NaSICON structure with R3̄c symmetry. These results also indicate the amorphous state of the coated carbon. The unit cell parameters of NVMFP/C are a = b = 8.7892 (2), c = 21.7124 (5) and V = 1452.55 (5) ų. Compared with those of NVP (a = b = 8.72 Å, c = 21.81 Å, V = 1436.46 ų),⁴¹ an increase in “a” cell parameter was observed, resulting in a larger cell volume than that of NVP. Table 1 provides the refinement details of the NVMFP/C. The unit cell of this material is illustrated in Fig. 1b, which is a three-dimensional open framework built from “lantern units” formed by edge-sharing PO₄ tetrahedra and VO₆/MnO₆/FeO₆ octahedra. Furthermore, two types of interstitial sites can host sodium atoms: Na1 site (100% occupied) with 6-fold coordination and Na2 site (80.5% occupied) with 10-fold coordination. The transition metals V, Fe and Mn share the 12c Wyckoff position with 75%, 12.5% and 12.5% occupancies, respectively. During the Rietveld refinement, the occupancies of Fe, Mn, and V sites were fixed to their nominal values after preliminary tests revealed that their simultaneous refinement caused divergence due to the strong parameter correlation, whereas individual refinement (with the other two fixed) was stable. Furthermore, this constraint improved the stability of the anisotropic displacement parameters. The elemental composition of Na, Mn and Fe was verified by Inductively Coupled Plasma (ICP) analysis (Table S1). The experimental Na/Fe and Mn/Fe ratios are in excellent agreement with those expected from the Rietveld refinement, confirming the stoichiometry of the synthesized material. The Bond Valence Sum (BVS) values (Brown & Altermatt)⁴² are in agreement with the oxidation states of P1, Na1, Na2, O1 and O2, with values of 5.174 (31), 0.872 (4), 1.063 (5), 2.129 (12) and 1.985 (17) for each atom, respectively. However, for the mixed Fe1/Mn1/V1 site, the BVS values are 2.786(14), 2.989(15), and 2.854(14), yielding a total BVS of 5.7247, which exceeds 5 valence units. This indirect interpretation may indicate the presence of other species with higher oxidation state, such as Fe³⁺ due to the reducing capability of the reducing agent, as observed in previous work.⁴³ Although the BVS analysis provides a strong indication of the electronic environment, further investigations using X-ray Photoelectron Spectroscopy (XPS) or Mössbauer remain necessary to unambiguously assign the oxidation states at the mixed V/Fe/Mn site. The details of atomic site, occupancy and BVSum obtained by Rietveld refinement are listed in Table 2. Geometric parameters are given in Table S2.

Fig. 1 (a) Rietveld refinement for the XRD pattern and (b) crystal structure visualization for NVMFP/C. (c) FT-IR and (d) Raman spectra of NVMFP and NVMFP/C. (e) Raman intensity mapping of the G and D bands, (f) I_D/I_G and I_G/I_D intensity ratio maps, (g) SEM, (h) TEM, (i) HR-TEM, and (j) EDS elemental mapping of NVMFP/C.

Table 1 Crystallographic data, data acquisition and structural refinement for NVMFP/C

Crystallographic data
Chemical formula	Na3.5V1.5Mn0.25F0.25(PO4)3
Mr (g mol^−1)	469.4887
Crystal symmetry	Trigonal, R-3̄c
X-ray source	Cu Ka (λ = 1.5406 Å)
Temperature (K)	298
a, c (Å)	8.78915 (14), 21.7124 (5)
V (Å^3)	1452.55 (5)
Z	6
Data acquisition
Diffractometer	Rigaku SmartLab
2θ values (°)	2θmin = 10, 2θmax = 100, 2θstep = 0.02
Refinement
Reliability factors	Rp = 4.387, Rwp = 5.517, Rexp = 4.385, RBragg = 3.856, χ^2 = 1.58
Refined parameters	94
Collected data points	4500

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Table 2 Atomic coordinates, displacement parameters (Ų), site occupancy rate and BVS calculation for NVMFP/C

Atom	Wyckoff position	x	y	z	Uiso	Occ. (<1)	BVSum
Na1	6b	0.00000	0.00000	0.00000	0.116 (5)		0.872 (4)
Na2	18e	0.6364 (6)	0.00000	0.25000	0.049 (3)	0.805 (7)	1.063 (5)
Fe1	12c	0.00000	0.00000	0.14767 (9)	0.0154 (8)	0.12501	2.786 (14)
Mn1	12c	0.00000	0.00000	0.14767 (9)	0.0154 (8)	0.12501	2.989 (15)
V1	12c	0.00000	0.00000	0.14767 (9)	0.0154 (8)	0.75000	2.854 (14)
P1	18e	0.2921 (3)	0.00000	0.25000	0.0159 (10)		5.174 (31)
O1	36f	0.1919 (4)	0.1686 (5)	0.0897 (2)	0.0134 (13)		2.129 (12)
O2	36f	0.0259 (6)	0.2106 (5)	0.19288 (16)	0.0266 (17)		1.985 (17)

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The FT-IR spectra of NVMFP and NVMFP/C materials are presented in Fig. 1c. Both samples exhibit identical band positions; however, slight differences in intensities are observed. The coated material exhibits high intensity, which is likely due to the improved infrared absorption conferred by the carbon layer.⁴⁴ The detected absorption bands correspond entirely to the vibration modes of the [PO₄]³⁻ ion: ν₃ (P–O) around 1175 cm⁻¹ and 1015 cm⁻¹; ν₁ (P–O) around 987 cm⁻¹; ν₄ (P–O) around 628 cm⁻¹, 577 cm⁻¹, and 532 cm⁻¹, and ν₂ (P–O) located at 452 cm⁻¹. These findings are in agreement with those reported in earlier studies.^34,45 Fig. 1d displays the Raman spectra of NVMFP and NVMFP/C, showing the presence of two peaks at 1350 cm⁻¹ (D-band) and 1590 cm⁻¹ (G-band), attributable to carbon, as well as an additional band at 1000 cm⁻¹ corresponding to the PO₄ stretching vibrations. The weak G and D bands observed in the NVMFP sample confirm the presence of minor residual carbon originating from the decomposition of citric acid and ethylene glycol during the calcination step. In contrast, the increased intensity of the carbon bands accompanied by the attenuation of the PO₄ band in NVMFP/C provides direct evidence of the higher carbon content in the composite.⁴⁶ To accurately quantify the structural evolution of the carbon coating, a Raman deconvolution of the D and G bands (inset in Fig. 1d and Table S3) was performed according to previous work.⁴⁷ The I_D4/I_G ratios (D₄ peak intensity/G peak intensity) for NVMFP and NVMFP/C samples are comparable. However, the I_D3/I_G ratio is higher for NVMFP than for NVMFP/C, while the I_D1/I_G ratio is approximately twice as high for NVMFP. Additionally, NVMFP/C exhibits the lowest Full Width at Half Maximum (FWHM) values for both the D₁ and G bands, with values of 77.6917 and 36.1320, respectively (resulting in D_1(FWHM)/G_(FWHM) ratio of 2.1502). These Raman results suggest that NVMFP/C possesses a more ordered carbon structure than NVMFP, as evidenced by its lower I_D1/I_G and I_D3/I_G ratios and narrower D₁ and G Raman bands, indicative of larger graphitic domains. It has been reported that the increase in the graphitic character of carbon, reflected by an increased sp² carbon content, is responsible for improved electrical conductivity through the formation of extended graphitic π-electron networks.^48–50 Raman mapping, shown in Fig. 1e and f, was employed to investigate the carbon distribution on the NVMFP/C grain surfaces by comparing the spatial intensities of the D and G bands, alongside both I_D/I_G and I_G/I_D intensity ratio maps. The overlapping and homogeneous distribution of G and D signals (Fig. 1e) confirms a uniform carbonaceous coating on the grain surfaces of NVMFP/C. The intensity ratio maps (Fig. 1f) reveal a predominance of graphitic character, although a small area exhibiting characteristic D-band features is observed, indicating localized structural disorder. The average carbon cluster size L_a of NVMFP/C calculated using the Tuinstra–Koenig method (Note S1) is 4.09 nm. The TGA measurements (Fig. S1b) revealed carbon contents of about 2 wt% and 2.87 wt% for NVMFP and NVMFP/C, respectively. The microstructure of NVMFP/C powder was investigated by electron microscopy (SEM and TEM). As displayed in Fig. 1g and h, the NVMFP/C particles have sizes ranging from 0.3 to 1 µm with irregular shapes. Lattice fringes with a spacing of 0.376 nm are visible in the HR-TEM image (Fig. 1i), which is consistent with the d_hkl value for the (113) plane of the NVMFP/C, indicating that the particles possess a high degree of crystallinity. In addition, the presence of an amorphous carbon shell approximately 3.5 nm thick was observed on the surface of the crystallized particles, which is in excellent agreement with the value calculated from the Raman spectra (∼4 nm). EDS mapping (Fig. 1j) indicates a uniform distribution of Na, V, Fe, Mn, O, C, and P throughout the NVMFP/C particles.

3.2. Electrochemistry

The electrochemical investigation of NVMFP/C and NVMFP cathode materials within the voltage window of 1.5–4.3 V vs. Na⁺/Na was examined using CR2032 coin cells (1C ∼ 114.2 mA g⁻¹ which is defined as 2 Na⁺ exchange in 1 hour). Electrochemical comparisons between NVMFP/C and NVMFP cathodes (Fig. S2) were performed based on the second galvanostatic cycle to ensure a complete evaluation of the redox transitions across the full potential range (1.5–4.3 V). This approach accounts for the capacity contributions below the open-circuit voltage, which are not fully captured during the initial charging process. A notable difference in specific charge capacity at 0.1C was observed between the two cathodes, with NVMFP/C exhibiting a higher charge capacity of approximately 167 mA h g⁻¹ (∼0.30 mA h cm⁻²) compared to 130 mA h g⁻¹ (∼0.23 mA h cm⁻²) for NVMFP. The enhanced capacity observed between the pristine and carbon-coated Fe-modified phase, notwithstanding a slight increment in the amount of carbon (from ∼2.0 wt% to ∼2.87 wt%), is due to the improved electronic conductivity achieved by the ex situ carbon coating, which promotes charge transfer within NVMFP cathode material, as evidenced by the Raman results. Fig. 2a demonstrates the first three charge/discharge cycles of NVMFP/C cathode at 0.1C. The electrode delivers initial charge and discharge capacities of 141 mA h g⁻¹ (∼0.25 mA h cm⁻²) and 167 mA h g⁻¹ (∼0.30 mA h cm⁻²), respectively. Two distinct charge/discharge voltage plateaus are observed at approximately 3.42 V and 4.0 V, matching well with the V⁴⁺/V³⁺, Mn³⁺/Mn²⁺ and V⁵⁺/V⁴⁺ redox couples, respectively.^33,51,52 An additional, smaller plateau appears near 2.62 V, consistent with the potential range of the Fe³⁺/Fe²⁺ redox activity in the NaSICON framework.^18,37,53 Moreover, a pseudo-plateau around 3.15 V is evident during charge, which may result from the extraction of Na⁺ from two distinct crystallographic sites within the NVMFP/C framework, a phenomenon previously reported in related studies.^25,27 In the second cycle, a new charge/discharge plateau emerges at around 1.7 V, aligning with the expected potential for the V³⁺/V²⁺ redox couple,^51,52 which provides an additional 20 mA h g⁻¹ to the charge capacity. Consequently, the total charge and discharge capacities are 167 mA h g⁻¹ (∼0.30 mA h cm⁻²) (extraction of approximately 2.94 Na⁺ ions per formula unit) and 164 mA h g⁻¹ (∼0.29 mA h cm⁻²), respectively, achieving a coulombic efficiency of 98.23% in the second cycle. The dQ/dV plot (Fig. 2b) derived from the second cycle of NVMFP/C cathode further corroborates the aforementioned cycling results by providing a clear signature of the redox transitions. Specifically, a pair of peaks at high potential is clearly evidenced, in good agreement with the V⁵⁺/V⁴⁺ transition. Additionally, shouldered and intense oxidation/reduction peaks were observed, which could correspond to the overlapping V⁴⁺/V³⁺ and Mn³⁺/Mn²⁺ couples, likely due to their similar redox potentials in the NaSICON framework. In contrast, a faint signal (inset in Fig. 2b), barely discernible from the baseline, could be attributed to the Fe³⁺/Fe²⁺ redox feature. Notably, peak splitting is observed at approximately 1.7 V during reduction and at 3.14 V during oxidation; these features can be attributed to the multi-step electrochemical insertion and extraction of Na⁺ into the NaSICON structure, as observed in related studies.^25,54 Remarkably, NVMFP/C electrode requires a presodiation treatment to achieve its full capacity. It is worth noting that the NVMFP/C electrode shows a capacity discrepancy of 26 mA h g⁻¹ between the first charge and discharge cycles, corresponding to approximately 0.45 moles of Na⁺. This surplus suggests a resodiation mechanism, where additional sodium ions are incorporated into the lattice to occupy the limited interstitial space of 4 Na⁺ available within the NaSICON framework,⁵⁴ which can lead to Na₄V_1.5Mn_0.25Fe_0.25(PO₄)₃ phase at a lower voltage ∼1.6 V. Consequently, the first-cycle irreversibility can be attributed primarily to Na inventory increase, where additional Na⁺ are inserted into the framework. Furthermore, the presodiation process can be mitigated through material optimization, notably by developing sodium-rich cathodes. However, this approach remains complex due to the need for precise stoichiometric control and demanding synthesis conditions. Alternatively, electrolyte optimization offers a strategy to regulate this phenomenon, particularly in full-cell configurations, through the incorporation of sacrificial sodium-source additives.^55,56 The cyclic voltammograms of the NVMFP/C electrode at 0.8 mV s⁻¹ (Fig. 2c) display four distinct redox peak pairs at 1.82/1.5 V, 3.33/2.99 V, 3.70/3.26 V, and 4.06/3.89 V attributed to the V³⁺/V²⁺, the split Na⁺ extraction phenomenon, the V⁴⁺/V³⁺(Mn³⁺/Mn²⁺) couples and the V⁵⁺/V⁴⁺ couple, respectively. However, the Fe²⁺/Fe³⁺ redox peaks are not clearly distinguishable in the CV curves, despite their contribution being evidenced by a smaller plateau in the galvanostatic test. A similar behavior was reported in a previous study.⁴³ The cycling stability of both high- and low-voltage regions was investigated over 50 cycles at 0.5C (Fig. S3). It is observed that both high- and low-voltage contributions remain significant throughout the cycling. However, a noticeable capacity fade was recorded, with a capacity retention of only 75%, which may be attributed to electrolyte decomposition resulting from prolonged exposure to both potential extremes. It is reported in the literature that, at high voltage, the Cathode Electrolyte Interphase (CEI) suffers from poor self-healing capability and dissolution, which negatively affects cycle life.^57,58 Notably, the coulombic efficiency of NVMFP/C at 0.5C remained stable and higher than 95% over 50 cycles, indicating that while some capacity is lost, the remaining redox processes maintain high reversibility and the parasitic reactions are relatively limited. These findings confirm the successful activation of multiple redox centers and their cyclability, highlighting the benefits of this specific stoichiometry.

Fig. 2 (a) Charge–discharge profile of NVMFP/C cathode at 0.1C, (b) dQ/dV curve of the second cycle for NVMFP/C at C/20, (c) CV curve of NVMFP/C electrode in the voltage window of 1.5–4.3 V at a scan rate of 0.8 mV s⁻¹, (d) rate capability of NVMFP/C cathode and (e) corresponding second-cycle charge–discharge curves, (f) the average voltage and first discharge capacity of NVMFP/C cathode, compared with other reported V-based NaSICON cathodes (at 1C) (for reference see Table S4), (g) cycle life of NVMFP/C cathode at 10C in the voltage range of 1.5–4.3 V.

The rate performance of the NVMFP/C electrode is shown in Fig. 2d. The NVMFP/C cathode provides initial discharge capacities of 146, 134, 119, 101, and 97 mA h g⁻¹, corresponding approximately to 0.27, 0.25, 0.22, 0.19 and 0.18 mA h cm⁻² at 0.5, 1, 2.5, 4 and 5C current rates, respectively. When the current rate was returned to 0.5C, the NVMFP/C electrode recovered a specific discharge capacity of 141 mA h g⁻¹ (∼0.26 mA h cm⁻²), indicating its remarkable reversibility and structural stability. The charge/discharge profiles with the increased rate as shown in Fig. 2e. It is clearly observed that the contribution of both high- and low-potential regions to the total capacity decreases as the C-rate increases. At 5C, the capacity is primarily sustained by the V⁴⁺/V³⁺ and Mn³⁺/Mn²⁺ redox regions. Interestingly, the NVMFP/C cathode exhibited an excellent energy density of 412 Wh kg⁻¹ at 1C, and is well-positioned among the highest values reported for V/Mn/Fe-based NaSICON systems (Fig. 2f and Table S4). The long-term cycling performance of the NVMFP/C cathode was evaluated at a high rate of 10C (Fig. 2g). The electrode delivered an initial specific capacity of 111 mA h g⁻¹ (∼0.22 mA h cm⁻²) and retained 82 mA h g⁻¹ (∼0.16 mA h cm⁻²) after 1400 cycles, corresponding to capacity retention of 74.34%. This excellent cycling stability highlights the robust structural integrity of the material, which enables highly reversible sodiation/desodiation within the wide voltage range of 1.5–4.3 V. To further evaluate the kinetic stability of the cell at 10C, the polarization during long-term cycling was analyzed (Fig. S4). The polarization decreases from 0.39 V at cycle 1 to 0.22 V at cycle 10, then gradually increases to 0.47 V at cycle 100, 0.74 V at cycle 1000, and reaches 1 V at cycle 1400. The initial decrease may be related to a stabilization or activation process during the early cycles. While the present results do not allow a definitive identification of the underlying mechanisms, the observed polarization growth could suggest cumulative interfacial occurring during prolonged cycling. Overall, the charge/discharge profiles exhibit minimal evolution during cycling that indicates well-preserved electrochemical kinetics. This is further supported by ex situ XRD (Fig. S6b) analysis after cycling, which confirms that the characteristic peaks of the NaSICON framework remain intact. The absence of structural degradation or secondary phase formation demonstrates the robust structural integrity of the NVMFP/C cathode during long-term cycling. Furthermore, ex situ XRD diagrams of the NVMFP/C electrode at different voltage stages during the first cycle at C/10 (Fig. S6a) demonstrate that the NaSICON structure undergoes reversible structural evolution, which confirms its structural stability during sodium insertion/extraction processes.

3.3. Charge storage behavior and Na-ion kinetics

The charge storage behavior and Na⁺ ion transport kinetics of NVMFP/C were evaluated using cyclic voltammetry (CV) at various sweep rates and the galvanostatic intermittent titration technique (GITT). From the cyclic voltammograms (Fig. 3a), the Na⁺ diffusion coefficient (D_Na⁺) and the charge storage behavior were analyzed by employing the following equations:

i_p = 2.69 × 10⁵n^3/2SD_Na^1/2C₀ν^1/2 (Randles–Sevcik equation) (1)

i_p = aν^b (2)

i = k₁ν + k₂ν^1/2 (3)

where i_p is the peak current (A), n is the number of electrons transferred, and S is the electrode surface area (0.79 cm²), D_Na⁺ is the diffusion coefficient (cm² s⁻¹), C_Na is the molar concentration of Na ion (2.39 × 10⁻² mol cm⁻³), ν is the scan rate (V s⁻¹), a, b, k₁, and k₂ are adjustable parameters that are determined by the fitting results between i_p and v.

Fig. 3 (a) CV curves of NVMFP/C electrode at various scan rates, (b) the current as a function of the square root of the scan rates, (c) log (current) versus log (scan rate), (d) pseudocapacitive contribution at various scan rates, (e) the capacitive contribution to the total current at 0.8 mV s⁻¹, and (f) and (g) the GITT curves for (f) charge and (g) discharge as a function of the Na⁺ concentration in Na_xV_1.5Mn_0.25Fe_0.25(PO₄)₃.

The diffusion coefficients D_Na⁺ were estimated using the Randles–Sevcik equation, assuming a reversible, diffusion-controlled electrochemical process based on the linear relationship between the peak currents (O1, O2, O3, R1, R2 and R3) and the square root of the scan rate (ν^1/2) as presented in Fig. 3b. The peak currents (O1, O2, O3, R1, R2 and R3) were determined at the maximum intensity of each identified redox process, as the well-defined nature of the peaks allowed for a consistent selection across all scan rates. The slope obtained from the linear fit, combined with eqn (1), yielded D_Na⁺ values (Table 3) ranging from 1.87 × 10⁻¹¹ to 5.38 × 10⁻¹² cm² s⁻¹, which is comparable to those of typical cathode materials with a NaSICON structure.^25,37 It is important to emphasize that the Randles–Sevcik equation used in these calculations only describes faradaic-controlled processes while neglecting any pseudocapacitive contributions. Nevertheless, the Na⁺ insertion/extraction mechanism can be rationalized by analyzing the b-value obtained from eqn (2). Depending on the value of b, two distinct charge storage behaviors can be distinguished: (i) a diffusion-controlled process when b ∼ 0.5, corresponding to semi-infinite ion diffusion in the solid-state host lattice, and (ii) a capacitive-controlled process when b ∼ 1.0, associated with surface or pseudocapacitive contributions. As shown in Fig. 3c and Table S5, the linear fit of log(i_p) versus log(ν) yields a slope (b) ranging from 0.49 to 1, which suggests a mixture of faradaic intercalation and pseudocapacitive mechanisms.⁵⁸ The contribution of each mechanism to the Na⁺ insertion/de-insertion in NVMFP/C electrode was evaluated using eqn (3). This equation contains two types of current: capacitive current (k₁ν) and diffusion-limited current (k₂ν^1/2). Fig. 3d displays the capacitive and the diffusion contributions versus scan rate (ν). At a scan rate of 0.8 mV s⁻¹, the capacitive process accounted for 63% of the total current (Fig. 3e). With increasing scan rate from 0.8 to 2 mV s⁻¹, the capacitive contribution increases from 63% to 73%. This clearly shows that capacitive behavior dominates the electrochemical reaction process of NVMFP/C cathode. Due to the minimal carbon content of our material, the capacitive dominant processes can be attributed to the small particle sizes and optimized interface, achieved by the Pechini approach, which facilitates surface-controlled reaction kinetics.^59,60

Table 3 Diffusion coefficient of redox peaks for the NVMFP/C cathode

Peak	R1	R2	R3	O1	O2	O3
DNa^+ (cm^2 s^−1)	5.58 × 10^−11	5.17 × 10^−11	5.38 × 10^−12	2.52 × 10^−11	9.78 × 10^−11	1.87 × 10^−11

---

The apparent diffusion coefficient of Na⁺ (D_Na⁺) was also calculated from GITT measurements using eqn (4). This method is considered more precise because it operates under near-equilibrium conditions, providing a more reliable measurement of the Na¹⁺ diffusion coefficients.

The parameter I corresponds to the applied current, while V_M denotes the molar volume of the electrode material (cm³ mol⁻¹). The symbol n designates the number of electrons involved in the charge-transfer process, and F refers to Faraday's constant (A s mol⁻¹). The term S represents the electrode surface area (0.79 cm²). The derivative dV/dx expresses the slope of the potential variation as a function of sodium composition (V mol⁻¹), whereas dV/d√t corresponds to the slope of the potential versus √t curve, obtained from the voltage–time profile (V s^−1/2). Fig. 3f and g show the charge/discharge GITT curves and the variation of the apparent diffusion coefficient as a function of Na⁺ concentration in Na_xV_1.5Fe_0.25Mn_0.25(PO₄)₃. During the charge/discharge process, the voltage plateau around 3.5 V exhibits a smaller overpotential, corresponding to an apparent diffusion coefficient of approximately 10⁻¹¹ cm² s⁻¹. However, when the voltage is either higher or lower than 3.5 V, the overpotential increases, and the apparent diffusion coefficient fluctuates between different values. At low voltages during discharge, a significant drop in the apparent diffusion coefficient was observed due to high occupation of vacancy sites by Na⁺ sodiation, which hinders further sodium insertion within the material's structure.¹³

EIS measurements were conducted on the NVMFP/C electrode after first charge and first discharge (Fig. 4). The inset in Fig. 4 displays the equivalent circuit used to fit this response, which consists of several components: a resistance (R_O) at a higher frequency, representing the ohmic resistance due to the electrolyte; a resistance (R_CEI) in parallel with a constant phase element (CPE_CEI) at intermediate frequency, describing the Cathode-Electrolyte Interphase (CEI) layer response; a resistance (R_ct) connected in series with a Warburg element (W), both of which are in parallel with a constant phase element (CPE_ct) at lower frequency. This last combination corresponds to faradaic impedance, where R_ct/CPE_ct is indicative of the charge transfer process, and the Warburg impedance (W) represents the diffusion of Na⁺ at the electrode–electrolyte interfaces. Table S8 summarizes the fitting parameters of NVMFP/C after first charge and first discharge. At the end of the first charge, the EIS plot shows a single semi-circle, which corresponds to the CEI layer response, followed by a diffusion line.⁶¹ At this stage, the R_CEI and R_ct resistance values are 143.4 and 2090 Ω, respectively. The higher R_ct prevents the formation of a distinct charge-transfer semi-circle. After the first discharge, the R_CEI and R_ct resistance show values of 550.2 and 1462 Ω, respectively, and an additional semi-circle appears. This new semi-circle corresponds to the charge transfer response and can indicate the domination of electrochemical intercalation reaction at this stage. This behavior was observed in previous work dealing with Li-ion system, in which a dominant intercalation–deintercalation process occurs.⁶² Moreover, the significant increase in R_CEI after the first discharge is a consequence of the CEI growth caused by the decomposition of the electrolyte.⁶³ Furthermore, R_ct remained relatively high at the end of both charge and discharge, indicating persistent interfacial limitations. The high value of R_ct may result from several combined factors, including the electrode thickness, which increases ionic transport pathways, and the electrolyte composition. Although NaClO₄ in EC : PC with FEC provides good bulk conductivity, FEC decomposition and high-voltage operation (4.3 V) can promote the formation of a resistive CEI layer, thereby increasing interfacial impedance.⁶⁴ Despite the relatively high Na⁺ diffusion observed by GITT within the NaSICON-type NVMFP/C framework, R_ct is mainly governed by interfacial charge-transfer processes, explaining why it remains in the kilo-ohm range.

Fig. 4 EIS spectra of NVMFP/C electrode after first charge/discharge at C/20.

4. Conclusion

In summary, we report the successful design and synthesis of Na_3.5V_1.5Mn_0.25Fe_0.25(PO₄)₃ (NVMFP/C), a novel V/Mn/Fe-based NaSICON-type cathode material, via the Pechini method followed by an ex situ carbon coating. Structural and morphological characterizations using XRD, FT-IR, Raman, TGA, SEM, EDS, and TEM confirm the formation of a highly crystalline, phase-pure trigonal NaSICON framework with uniform particle distribution and homogeneous elemental composition. The electrochemical evaluation of NVMFP/C as a cathode material for SIBs demonstrates a high reversible capacity of 167 mA h g⁻¹ (∼0.30 mA h cm⁻²) within a wide voltage window of 1.5–4.3 V at 0.1C, enabled by the activation of multi-electron redox reactions. The material achieves a remarkable energy density of 412 Wh kg⁻¹ at 1C, good rate capability, and remarkable long-term cycling stability, retaining 74% of its initial capacity after 1400 cycles at 10C. These outstanding performances are attributed to a combination of rapid Na⁺ diffusion through the robust 3D framework, enhanced electronic conductivity and structural integrity during repeated sodiation/desodiation achieved by the synergy between Fe-incorporation, Pechini-derived nanostructure, and the low-cost ex situ carbon coating. Kinetic studies using cyclic voltammetry, GITT, and EIS reveal a significant capacitive contribution, indicating that surface-controlled processes play a key role in facilitating fast charge storage. The synergy between bulk diffusion and pseudocapacitive kinetics underpins the superior rate performance and high-energy output of NVMFP/C. This work highlights that careful compositional engineering, multi-electron redox activation, and microstructural optimization can be strategically combined to develop NaSICON-type cathodes with high energy density, rapid kinetics, and long-term durability.

Author contributions

Loubna Chayal: data curation, formal analysis, investigation, methodology, writing – original draft, Soraia El Moutchou: data curation, software, Noha Sabi: data curation, formal analysis, Hicham Ben Youcef: project administration, resources, Hasna Aziam: supervision, validation, writing – review & editing, Mohamed Saadi: supervision, validation, project administration, resources, and Mohammed Hadouchi: supervision, methodology, validation, writing – review & editing, project administration, resources.

Conflicts of interest

There are no conflicts of interest to declare.

Data availability

Data are available in the supplementary information (SI) or from the corresponding author upon reasonable request. Supplementary information: notes S1, S2 and S3, Fig. S1, S2, S3, S4, S5 and S6, and Tables S1, S2, S3, S4, S5, S6, S7 and S8. See DOI: https://doi.org/10.1039/d6ra00418k.

Acknowledgements

Financial support from the CNRST was provided through the “PhD-Associate Scholarship – PASS” Program (74 UM5R2023). The authors sincerely thank UATRS-CNRST for facilitating access to the transmission electron microscope. Loubna Chayal also acknowledges UM6P for financial support during her research stay at HTMR-UM6P.

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