Stability and Scalability of Na₃V₂(PO₄)₃ Cathode Material for Next-Generation Sodium-Ion Batteries

Introduction

Sodium-ion batteries (SIBs) are a viable alternative to lithium-ion batteries (LIBs) due to the availability and affordability of sodium resources, particularly for large-scale energy storage systems. The materials utilized for SIBs are not only abundant but also safe and economical, This makes them very appealing for real-world uses in energy storage technologies that are renewable.^1-3 The markets for electric cars and portable devices are dominated by LIBs because of their high energy density, long cycle life and safety; nevertheless the rising costs and limited availability of lithium reserves pose significant challenges for large-scale implementation.^4,5 Therefore, the development of alternate energy storage systems that provide comparable performance at reduced costs. In this context, SIBs have garnered increasing attention as a promising solution. Leveraging sodium’s widespread availability and low cost, researchers have made substantial progress in improving the efficiency, cycling stability, and overall performance of SIBs.^6,7 Notably, NASICON-structured materials such as Na₃V₂(PO₄)₃ have demonstrated great potential as advanced cathode materials, owing to their superior electrochemical stability and ion conductivity. Moreover, innovations in doping techniques and material fabrication, including improving the Na-ion conductivity of electrolytes based on NASICON, have further driven the development of high-performance SIBs.  Despite the progress made in sodium-ion batteries (SIBs), their practical application remains challenging. One of the major obstacles is Finding appropriate electrode materials, especially cathode materials, that can offer quick ion diffusion paths for steady Na⁺ insertion and extraction is challenging due to the higher ionic radius of Na⁺ than Li⁺ (0.98 Å vs. 0.69 Å).^8,9 The structural degradation that occurs during the intercalation and deintercalation processes is exacerbated by this difference in ionic size. and lowers the ion diffusion coefficient, further complicating the development of high-performance electrode materials.  Identifying electrode materials with superior performance is significantly more challenging for SIBs than for lithium-ion batteries (LIBs). The larger size of Na⁺ not only hampers the diffusion kinetics but also increases the stress on the electrode structure, leading to rapid capacity fading and reduced cycling stability. Consequently, achieving durable and efficient SIBs requires innovative approaches to material design and optimization to overcome these inherent limitations.^10,11

The use of layered transition metal oxides^12,13, metal hexacyanometalates ^14,15, and polyanionic compounds ^16,17 as cathode materials has been widely investigated in recent years. Among these, Na₃V₂(PO₄)₃ (NVP) stands out due to its sodium superionic conductor (NASICON) framework, resulting in superior ionic conductivity by facilitating 3D tunnels for Na⁺ insertion/extraction. Additionally, the NVP structure provides high thermal stability, a fast Na⁺ diffusion coefficient, and superior structural stability during cycling, making it an ideal candidate for sodium-ion batteries with superior performance. Polyanionic compounds and multilayer metal oxides are examples of cathode materials that have drawn significant attention because of its long cycle life and superior thermal stability. However, organic compounds have low reaction kinetics, and metal oxides frequently undergo charge/discharge cycles that result in irreversible structural alterations. However, polyanionic substances such as NVP, have exceptional electrochemical performance, great structural stability, and rapid Na⁺ diffusion.^20-24 Among the cathode materials investigated, NVP is particularly promising due to its unique NASICON framework, It facilitates effective Na+ diffusion by offering a 3D open network. NVP inserts and removes two sodium ions during the electrochemical process, producing an impressive 3.4 V peak voltage and 117 mAh g⁻¹ specific capacity [18,19]. Furthermore, NVP demonstrates a higher energy density of approximately 400 Wh kg⁻¹ and remarkable cycling stability, solidifying its capacity as a top cathode substance for SIBs^20-24.

The polyanionic molecular structure Na₃V₂(PO₄)₃ (NVP) has NASICON (Na Super Ion Conductor) structure, This makes it a suitable sodium-ion battery cathode component. NVP’s distinct three-dimensional structure, formed by corner-sharing VO₆ octahedra and PO₄ tetrahedra, contributes to excellent structural stability and extended cycle life by offering ample interstitial gaps for effective Na-ion diffusion.^25-26 While the sodium ions at Na1 sites maintain a strong interaction with the oxygen atoms, the sodium ions at Na2 sites actively engage in the charge/discharge process.^27,28 NVP’s intrinsic disadvantages, including poor rate performance and weak ionic conductivity, persist despite these benefits, require further optimization to meet practical energy storage demands.²⁹ In this study, we synthesize and characterize Na₃V₂(PO₄)₃/C (NVP) as a material for the cathode of sodium-ion batteries (SIBs). A sol-gel technique was used to prepare three NVP samples: pristine NVP-1, stored NVP-2 to assess structural stability over time, and bulk NVP-3 (10× scale) for large-scale production analysis. The results of this study help to develop the advanced, cost-effective and high performing cathode materials for sodium-ion batteries.

Experimental section

Chemicals

The electrode material has been synthesized using analytical-grade chemicals, including citric acid, sodium carbonate anhydrous (Na₂CO₃, >99.5%), ammonium phosphate monobasic (NH₄H2PO₄, >98%), and vanadium(V) oxide (V₂O₅, >98%).

Material synthesis

Using the Sol-Gel technique, the porous Na₃V₂(PO₄)₃/C (NVP) material was created. Using the standard procedure, a stoichiometric mixture of sodium carbonate (Na₂CO₃), vanadium pentoxide (V₂O₅) and ammonium dihydrogen phosphate (NH₄H₂PO₄) was used in molar ratios of 3: 2: 6. Initially, 0.6359 g of Na₂CO₃ and 1.3804 g of NH₄H₂PO₄ were dispersed to create Solution A in 20 ml of deionized water., which was stirred at 70°C for 15 minutes. 1.5370 g of citric acid and 0.7275 g of V₂O₅ were dissolved simultaneously in 20 ml of deionized water to generate Solution B. The mixture was then stirring for 30 minutes at 70°C until it was fully dissolved. A dark blue colloid was produced by progressively adding Solution A to Solution B while stirring constantly for five hours at 80°C. To create a dry precursor gel, this mixture was dried for 24 hours at 80°C. The dehydrated item was ground and calcined under flowing argon gas at 750°C for 6 hours. Once naturally cooled to ambient temperature, the final composite was ground into a fine powder. To confirm the scalability and reproducibility of this synthesis process, a bulk synthesis was performed using the same molar ratios but with tenfold increased quantities of each reagent, maintaining identical conditions for stirring, drying, and calcination.

Figure 1: Scheme of synthesis of NVP sample by sol-gel method. (This work). Click here to View Figure

Characterization

Electrodes NVP-1, NVP-2, and NVP-3 were structurally and morphologically characterized utilizing a variety of analytical methods. To verify the creation of the NASICON-type structure, Utilizing a Rigaku SMARTLAB system operating at 40 kV voltage and 50 mA current, XRD patterns were recorded using Cu Kα radiation (λ = 1.54184 Å).  A Nova Nano FE-SEM 450 was used to analyze the composition and surface morphology, revealing nano-sized particles and flake-like structures with time-dependent changes in porosity. Thermogravimetric analysis (TGA) was performed with a Hitachi model-7300 to evaluate thermal stability, showing minimal weight loss due to moisture and carbon content. Using a Thermo Scientific Nexsa G2 equipment, The oxidation states and chemical composition of the elements inside the NVP structure were determined using XPS.

Results and Discussion

X-ray diffractogram analysis

The structural characteristics of the synthesized Na₃V₂(PO₄)₃/C (NVP) were investigated using XRD. The development of the rhombohedral NASICON-type framework with the R3c space group was verified by indexing the diffraction patterns. Fig. 2(a) shows the diffractograms that were gathered. The prominent peaks at 2θ values of approximately, 14.34°, 20.21°, 23.91°, 28,91°,  31.74°, and 32.26°, 35.81°, 43.51°, 48.44°,  49.18° correspond to the (012) (104) (113) (024) (211) (116) (300) (1010) (315) (042) planes, respectively, With a rhombohedral NASICON framework, The R-3c space group is closely linked to all of the prominent diffraction peaks, confirming the Na₃V₂(PO₄)₃ phase. These findings align with the published data (JCPDS 00-062-0345) [30]. The comparison of XRD patterns between NVP-1 and NVP-2 reveals a noticeable shift in the peak positions, particularly for prominent reflections corresponding to the (113), (024), and (116) planes. In NVP 1, these peaks are observed at 23.91°, 28.91°, and 32.26°, while in NVP 2, they appear at 24.04°, 29.03°, and 32.28°, respectively. These small but measurable shifts suggest a change in the lattice parameters, This may be due to the material’s gradual structural change or degradation over storage. This shift in peak positions is typically associated with minor lattice distortions rather than a complete phase transformation. The preservation of the overall peak pattern and indexing to the same rhombohedral NASICON-type framework (R-3c space group) confirms that the material maintains its original phase, suggesting structural stability at a macroscopic level. The XRD pattern of the bulk NVP material, synthesized for large-scale use, shows prominent peaks at 23.77°, 28.73°, and 32.04°, In accordance with the planes (113), (024), and (116). The XRD pattern remained consistent with the standard synthesis, confirming the preservation of phase purity and crystal structure integrity during scale-up. Bulk materials often experience microstructural strain or defects due to larger processing volumes, which may impact ionic conductivity and phase stability. Nonetheless, the retention of key structural features confirms that the bulk material retains the essential Na₃V₂(PO₄)₃ phase, making it suitable for practical applications.

To determine the average crystallite size of the synthesized NVP samples, the Debye-Scherrer formula was utilized. The formula is expressed as:

where θ is the angle of diffraction, λ is the X-ray wavelength, and β is the full width half maximum in radians. The calculated average crystallite size for the most intense peak was derived for the NVP samples using their respective FWHM values. The average particle size for NVP-1, NVP-2, and NVP-3 was calculated as 44.22 nm, 40.34 nm, and 49.79 nm, respectively, based on the FWHM values of their most intense peaks.

Table 1: Crystal structure parameters of synthesized NVP materials.

Materials	Lattice Parameters		d spacing for (116)	Crystallite size by XRD	FWHM(116)
	a = b (Å)	c (Å)	(Å)	(Å)	(◦)
NVP-1	8.68042	21.8194621.62472 21.79506	2.7706	461	0.187
NVP-2	8.67731		2.7693	422	0.205
NVP-3	8.72238		2.7893	521	0.166

Figure 2: (a) XRD patterns of NVP-1, NVP-2, NVP-3 (b) expanded view of the (1 1 3), (0 2 4), and (1 1 6) diffraction peaks. Click here to View Figure

Table 2: Based on the lattice constants, the following structural parameters were calculated

Parameter	NVP-1	NVP-2	NVP-3 (Bulk)	JCPDS 00-062-0345
c/2a	1.246	1.257	1.249	1.249
Tetragonal Distortion(D =2-(c/a))	-0.492	-0.514	-0.499	-0.498
Volume (ų)	1410.105 ų	1430.187 ų	1436.01 ų	1436.17ų

The c/2a ratio of 1.246, 1.257, 1.249 is within the expected range for NVP and indicates good symmetry and minimal lattice distortion, which is favourable for sodium ion diffusion. The calculated tetragonal distortion values of D = -0.492, -0.514, -0.499 suggest a near-ideal structure. While D ideally should be close to zero to indicate no distortion, the minor negative values observed imply slight lattice distortions. However, the negative value suggests some degree of lattice distortion. For the bulk sample (NVP-3), the c/2a ratio and tetragonal distortion closely align with the JCPDS reference values, reflecting a well-maintained structural symmetry and minimal deviations from the standard NASICON framework. The unit cell volume of the bulk NVP (1436.01 ų) is almost identical to the JCPDS reference value (1436.17ų), indicating a stable and well-ordered crystal structure.

Thermal gravimetric analysis

The NVP samples were subjected to thermogravimetric analysis (TGA) in a nitrogen environment at a scan rate of 15.0°C/min between 27°C and 600°C, as illustrated in Fig. 3. Between 100°C and 200°C, the first weight loss that was noticed was approximately 1.25%, 2.19% and 1.35% for the respective samples, The elimination of water molecules that are weakly bonded or physically adsorbed is the reason behind this. A subsequent weight loss of about 1.11%, 1.34% and 1.10% occurred between 250°C to 500°C, indicating the combustion of carbon species and the partial evacuation of water that was trapped inside the nanoparticles. The weight loss continues at a decreased rate between 400°C and 600°C, which most likely reflects the elimination of residual water from the nanoparticle structure and further combustion of carbon material.³⁰

The thermal stability of the NVP samples varies, according to TGA. NVP-1 exhibits the lowest initial weight loss of 1.25%, followed by 1.35% for NVP-3 (Bulk) and the highest 2.19% for NVP-2. The time-delayed sample (NVP-2) shows higher weight loss due to prolonged exposure to environmental conditions, leading to increased absorption of water and potentially more carbon contamination. The subsequent reduction in between 250°C to 500°C is similar for all samples, ranging from 1.10% to 1.34%, reflecting partial removal of water and carbon. However, NVP-3 shows slightly higher overall weight loss, suggesting a greater content of adsorbed water and residual carbon. The thermal stability of NVP-1 and NVP-3 (Bulk) is relatively comparable, while NVP-2 demonstrates a slightly reduced stability due to its higher moisture content.

Figure 3: TGA curve of NVP-1, NVP-2, NVP- (Bulk).  Click here to View Figure

SEM analysis

The synthesized NVP sample’s surface morphology was examined utilizing Nova Nano FE-SEM 450 at a magnification of 20,000x. NVP-1 and NVP-3 displays a combination of nano-sized particles and flake-like structures. The nanoparticles exhibit irregular shapes, while the flake structures show thin, layered formations. The agglomeration of particles is noticeable, indicating a tendency for clustering, which could influence the material’s electrochemical performance by affecting the active surface area. The NVP-2 shows a transition from distinct nano-flakes to a more compact, porous structure with smoother particles. This change is due to particle aggregation and structural rearrangement over time. Increased porosity enhances electrolyte penetration, while particle aggregation and structural rearrangement affect surface area and ion transport, influencing electrochemical performance.

Figure 4: SEM images of (a) NVP-1 (b) NVP-2 (c) NVP-3.  Click here to View Figure

The estimated particle sizes of all three samples range from 40 to 50 nm, indicating relatively small dimensions that contribute to an increased surface area. A few nanoparticles exhibit slight agglomeration, forming small clusters. The calculated average particle sizes are 82.21 nm for NVP-1, 78.28 nm for NVP-2, and 85.04 nm for NVP-3. Notably, the average particle size of NVP-2 decreased after six months, suggesting a time-induced effect on the material’s morphology, as further confirmed by XRD analysis. The homogeneous composite’s production is confirmed by the EDS mapping of NVP-1, which reveals that the particles Na, V, P, and O are uniformly distributed. 

Figure 5: Elemental composition mapping NVP-1. Click here to View Figure

X-ray photoelectron spectroscopy (XPS)

In order to further examine the element’s oxidation states and chemical composition in the Na₃V₂(PO₄)₃ (NVP-1). The results of X-ray photoemission spectroscopy (XPS) are displayed in Fig. 6. The Na 1s peak at 1071.4 eV confirms sodium ions in the structure, while the P 2p peak at 133.3 eV corresponds to phosphorus in the +5 oxidation state within phosphate groups. The oxygen in PO₄³⁻ is liable for the O 1s peak at 531.0 eV, while adventitious carbon pollution is the cause of the C 1s peak at 285.0 eV. The V 2p spectrum exhibits peaks at 523.5 eV and 516.6 eV, corresponding to V 2p_1/2​ and V 2p_3/2​, confirming vanadium in the +3 oxidation state. This analysis verifies the formation of a stable vanadium phosphate framework, essential for efficient sodium-ion storage and transport.^31-33

Figure 6: XPS spectra (A) full spectra of NVP (B) V 2p of NVP sample (C) Na 1s NVP sample, (D) P 2p of NVP sample, (E) O 1s NVP sample (F) C 1s NVP sample. Click here to View Figure

Conclusion

In conclusion, Na₃V₂(PO₄)₃/C (NVP) materials were successfully synthesized using the sol-gel method, a cost-effective and scalable technique suitable for large-scale production. XRD confirmed the formation of a rhombohedral (NASICON-type) structure with excellent phase purity. The structural stability of NVP-2 demonstrates minor lattice distortions over time, while bulk NVP-3 maintains phase integrity for practical applications. TGA analysis indicates good thermal stability with minimal moisture and carbon content variations. SEM analysis reveals nano-sized particles with time-induced morphological changes affecting porosity and ion transport. XPS verified the presence of Na, P, O, and V in appropriate oxidation states, forming a stable vanadium phosphate framework essential for efficient sodium-ion storage. The sol-gel method’s precise control over composition, uniform particle distribution, and reproducibility makes it highly effective for synthesizing superior materials for sodium-ion batteries on a commercial scale.

Acknowledgment

The authors want to acknowledge the University Grants Commission, New Delhi, India for the Ph.D. fellowship and MRC (MNIT, Jaipur), Manipal University (Jaipur), and the Department of Physics (University of Rajasthan, Jaipur) for assisting with the samples’ physical characterizations. 

Funding Sources

There is no funding Sources

Conflict of Interest

The authors declare no conflicts of interest regarding the research, authorship, or publication of this paper.

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