Sol–Gel Synthesis of Carbon-Containing Na₃V₂(PO₄)₃: Influence of the NASICON Crystal Structure on Cathode Material Properties

Abstract

With the rapid advancement of energy storage technologies, there is a growing demand for affordable, efficient, and environmentally benign battery systems. Sodium-ion batteries (SIBs) present a promising alternative to lithium-ion systems due to sodium’s high abundance and similar electrochemical properties. Particular attention is given to developing NASICON -sodium (Na) super ionic conductor, type cathode materials, especially Na3V2(PO4)3, which exhibits high thermal and structural stability. This study focuses on the sol–gel synthesis of Na₃V₂(PO₄)₃ using citric acid and ethylene glycol, as well as investigating the effect of annealing temperature (400–1000 °C) on its structural and electrochemical properties. Phase composition, morphology, textural characteristics, and electrochemical performance were systematically analyzed. Above 700 °C, a highly crystalline NASICON phase free of secondary impurities was formed, as confirmed by X-ray diffraction (XRD). Microstructural evolution revealed a transition from a loose amorphous structure to a dense granular morphology, accompanied by changes in specific surface area and porosity. The highest surface area (67.40 m²/g) was achieved at 700 °C, while increasing the temperature to 1000 °C caused pore collapse due to sintering. X-ray photoelectron spectroscopy (XPS) confirmed the predominant presence of V³⁺ ions and the formation of V⁴⁺ at the highest temperature. The optimal balance of high crystallinity, uniform elemental distribution, and stable texture was achieved at 900 °C. Electrochemical testing in a Na/NVP half-cell configuration delivered an initial capacity of 70 mAh/g, which decayed to 55 mAh/g by the 100th cycle, attributed to solid-electrolyte interphase (SEI) formation and irreversible Na⁺ trapping. These results demonstrate that the proposed approach yields high-quality Na₃V₂(PO₄)₃ cathode materials with promising potential for sodium-ion battery applications.

1. Introduction

In the context of global transformations in energy resource structures and the rapid development of electric vehicles, efficient and environmentally friendly energy storage technologies in battery systems have become a key factor for sustainable development [1,2]. Over the past three decades, lithium-ion batteries (LIBs) have significantly contributed to modern society’s progress due to their high energy density and extended service life [3]. However, lithium’s high cost and uneven distribution in Earth’s crust (0.0065% abundance) are driving the search for more economical and accessible alternative materials [4,5]. Sodium-ion batteries (SIBs) are considered a promising alternative to LIBs. Their performance characteristics are largely similar to LIBs, while sodium has a significantly higher crustal abundance—approximately 2.75% compared to 0.0065% for lithium—ensuring potential availability and reduced production costs [6,7]. This makes SIBs an attractive solution for large-scale applications.

Particular attention in SIB development is given to NASICON-type polyanionic cathode materials, which feature a three-dimensional structure with extensive ionic diffusion channels, high operating potential, and superior structural stability [8,9,10]. Among these, Na₃V₂(PO₄)₃ (NVP) stands out as one of the most promising cathodes due to its robust open framework that maintains stability during sodium-ion extraction, enabling efficient reversible interaction with Na⁺ ions [11,12]. The NVP cathode demonstrates outstanding electrochemical performance: a reversible specific capacity of ~117 mAh/g (enabled by a two-electron reaction), a high operating voltage of ~3.4 V vs. Na⁺/Na, and excellent thermal stability associated with the V³⁺/V⁴⁺ redox couple [13,14].

However, the synthesis technologies for such materials require optimization. Analysis of existing Na₃V₂(PO₄)₃ synthesis methods reveals that each approach has specific advantages and limitations [15,16]. The hydrothermal method promotes the formation of highly crystalline crystals with a porous structure, improving electrochemical performance, but it involves high equipment costs and scaling difficulties, limiting industrial applications [17,18]. The solid-state method, traditionally used for NVP preparation, suffers from component inhomogeneity, unwanted impurity formation, and uncontrolled morphology, which negatively affects electrochemical properties and increases production costs [19,20,21]. The sol–gel method enables the production of materials with high homogeneity and precise composition control at relatively low temperatures but requires long reaction times and subsequent thermal treatment [22].

As an innovative approach, the glycol-citrate method—a modification of traditional sol–gel synthesis based on citric acid and ethylene glycol [23,24]—is being considered. These reagents form complex ester-like structures that promote molecular mixing of components and uniform cation distribution in the precursor. This method enables low-temperature formation of the target phase, facilitates in situ carbon coating when using organic precursors (e.g., sucrose or PVA), and allows for obtaining nanoparticles with narrow size distribution, stable morphology, and low porosity.

Despite the widespread recognition of NVP as a promising cathode material, there is still no systematic study in the literature devoted to a comprehensive analysis of the effect of annealing temperature on all aspects of its formation and functionality. Filling this gap is a key goal of our work. We present a detailed study of how variation in this important synthesis parameter controls phase purity, particle morphology, and as a result, the electrochemical parameters of NVP. Thus, the novelty of our approach lies in the establishment of an exhaustive set of structure-property relationships, which is unattainable without a focused and detailed systematic study.

This work presents a novel sol–gel synthesis approach for Na₃V₂(PO₄)₃ using citric acid and ethylene glycol as dual-function organic complexing agents, offering improved control over precursor homogeneity and in situ carbon network formation. The synthesized materials were systematically annealed across an extended temperature range (400–1000 °C) to establish comprehensive structure-property relationships. The obtained samples were thoroughly characterized using X-ray diffraction (XRD) for phase analysis, scanning electron microscopy with energy-dispersive spectroscopy (SEM/EDS) for morphological and elemental studies, nitrogen adsorption–desorption (BET) for surface area and porosity evaluation, infrared spectroscopy (FT-IR) for functional group identification, and electrochemical testing for performance assessment. The broad thermal treatment range combined with multi-technique characterization provides unique insights into the material’s evolution and optimization potential for sodium-ion battery applications.

2. Materials and Methods

2.1. Chemicals

The Na₃V₂(PO₄)₃ sample was synthesized via a precisely designed sol–gel method using the following reagents: sodium dihydrogen phosphate (NaH₂PO₄), ammonium vanadate (NH₄VO₃), citric acid (C₆H₈O₇), and ethylene glycol (C₂H₆O₂).

The Na₃V₂(PO₄)₃ sample was synthesized through a precisely controlled sol–gel process using sodium dihydrogen phosphate (NaH₂PO₄), ammonium vanadate (NH₄VO₃), citric acid (C₆H₈O₇), and ethylene glycol (C₂H₆O₂) as organic complexing agents. The dissolved vanadium salts and sodium dihydrogen phosphate in a 1:1 molar ratio were mixed under continuous stirring, followed by addition of citric acid in a 1:2 salt-to-acid molar ratio, during which the solution color changed from yellow to deep blue, indicating the reduction of V⁵⁺ to V⁴⁺ by citric acid. Ethylene glycol was then added in a 1.1:1 ratio to citric acid, and the pH was carefully adjusted to 2.5–3.0 using NH₄OH/HNO₃ before heating at 80 °C for 4 h until a viscous gel formed. The gel was subsequently heated at 250 °C for 2 h to obtain a black porous precursor, which was then subjected to a two-stage calcination process: first at 400 °C for 1 h in static air to remove organic components and form an amorphous structure, followed by grinding and annealing under argon atmosphere (100 sccm flow rate) at incrementally increasing temperatures from 700 °C to 1000 °C (in 100 °C steps) for 1 h at each temperature with a controlled heating rate of 10 °C/min between steps. It is important to note that each sample was annealed directly from the initial state (after 400 °C), rather than successively increasing the temperature. As a result, the following series of samples were obtained: 400, 700, 800, 900 and 1000 °C. This optimized synthesis protocol enabled molecular-level precursor homogeneity while facilitating controlled polycondensation reactions essential for forming the desired NASICON-type crystalline structure, with the intermediate 400 °C calcination step preventing carbon loss while effectively removing organic residues, and the subsequent argon atmosphere annealing preserving the vanadium oxidation state while allowing controlled crystallite growth.

2.2. Physico-Chemical Methods

The structural characterization of materials was performed using a Shimadzu XRD-7000S laboratory diffractometer (Shimadzu, Kyoto, Japan) with CuK_α1-K_α2 radiation (40 kV, 30 mA, average wavelength λ = 1.5418 Å), Ni Kβ filter, scanning range of 5–60°, step size of 0.02°, and counting time of 0.6 s per point. Phase identification was carried out by X-ray phase analysis using the PDF-2.0 powder diffraction database in CSM software V9.10.0, while lattice parameter refinement was performed using the Rietveld method in GSAS (General Structure Analysis) software 3.0 [25]. Also, for qualitative analysis of X-ray diffraction data, ICDD ICDD PDF-4+ 2023 database were used along with COD database.

The average crystallite size (LXRD, nm) was determined from the XRD results using Scherrer’s formula (FWHM values determined by fitting XRD data in Fityk software):

$$\mathrm{LXRD} ={\displaystyle \frac{\mathrm{K} \mathsf{\lambda }}{\mathrm{FWHM} \cos \mathsf{\theta }}}$$

(1)

where K—crystallite shape factor, FWHM—full weight of XRD reflection at half maximum, $\mathsf{\theta }$– X-ray diffraction angle, (rad), $\mathsf{\lambda }$—X-ray wavelength (0.154 nm).

Morphological characterization was conducted using a Carl Zeiss ULTRA 55 Plus scanning electron microscope (Carl Zeiss, Oberkochen, Germany), with elemental composition determined by Electron Probe Microanalysis (EPMA) using an Oxford X-Max 80 detector (Oxford Instruments, Abingdon, UK) and Energy Dispersive X-ray Spectroscopy (EDS) on a Shimadzu EDX-7000P spectrometer (Shimadzu, Kyoto, Japan). Particle size analysis was performed using ImageJ software v1.54p by measuring 200 particles in four directions per particle, with data processed into frequency histograms and fitted with Gaussian functions to determine average particle sizes.

Textural properties were evaluated through low-temperature nitrogen adsorption measurements on an Autosorb-iQ analyzer (Quantachrome, Boynton Beach, FL, USA), with specific surface area calculated by the BET method and pore size distribution modeled using DFT. FTIR spectra were acquired on an Iraffinity-1S spectrophotometer (Shimadzu, Kyoto, Japan) using nujol mulls between KRS-5 crystal plates, with subsequent spectral processing to remove characteristic nujol absorption bands (720 cm⁻¹, 1372 cm⁻¹, 1450 cm⁻¹, 2913 cm⁻¹, and 2847 cm⁻¹) as referenced in prior studies [26].

X-ray photoelectron spectroscopy (XPS) measurements were performed on a Specs spectrometer with Phoibos analyzer (150 mm radius) using AlKα radiation (1486.7 eV) at the NanoFES station of the Kurchatov Synchrotron and Neutron Research Complex. Energy calibration was referenced to the C 1s peak at 285.0 eV, with survey spectra collected at 1 eV steps and high-resolution spectra at 0.1 eV steps (pass energy 50 eV), processed using CASA XPS software (Casa Software Ltd., Devon, UK) [27].

Electrochemical testing employed two-electrode half-cells with Na₃V₂(PO₄)₃ working electrodes, sodium metal counter electrodes (Sigma Aldrich, St. Louis, MO, USA), and glass fiber separators (hydrophilic GF/A 2916, density 66 g/m², thickness 0.29 mm), using 1M NaPF6 in EC/PC (1:1 vol.) electrolyte (Kishida Chemical, Osaka, Japan) of 1 mL. The sample was mixed with polyvinylidene difluoride (PVDF) and carbon black binder in a weight ratio of 85:15 and dispersed to form a homogeneous slurry. This slurry was cast onto aluminum foil, which was subsequently calendared at 70 °C and punched into 15 mm-diameter electrodes. Each electrode was weighed and then dried under vacuum at 100 °C for 12 h prior to transfer into an argon-filled glovebox; areal loading 176.7 mg cm⁻². Galvanostatic measurements were conducted using an Elins P-20X8 (Smart-Stat, Chernogolovka, Russia) potentiostat/galvanostat with ES8 software V2.05 (Smart-Stat, Chernogolovka, Russia), at room temperature of 25 °C.

3. Results and Discussion

3.1. Characterization of Obtained Materials

X-ray phase analysis of powders synthesized at different temperatures (700 °C, 800 °C, 900 °C) confirms the formation of a NASICON-type structure (Figure 1). The XRD patterns reveal a gradual development of crystalline Na₃V₂(PO₄)₃ phase with increasing annealing temperature. The diffractogram of the sample annealed at 400 °C shows a broad amorphous halo in the 2θ range of 20–35°, indicating predominantly amorphous or weakly crystallized structure. Distinct diffraction peaks characteristic of Na₃V₂(PO₄)₃ phase (JCPDS №53–0018) first appear at 700 °C, marking the onset of material crystallization. With further temperature increase (800–1000 °C), the peaks become progressively sharper and more intense, reflecting enhanced crystallinity and crystallite growth. All samples annealed above 700 °C demonstrate excellent agreement between experimental peaks and the reference Na₃V₂(PO₄)₃ phase pattern, confirming the formation of pure phase without detectable impurities.

The observed reflection conditions (hkl: −h + k + l = 3n; 00l: l = 6n) are characteristic of the R$\stackrel{\mathrm{-}}{3}$c (№ 167) space group. The refined unit cell parameters obtained through Rietveld analysis are presented in

Table 1. Structural parameters of the investigated materials.

Sample	Identified Phase	Unit Cell Parameters, Å		Calculated Density, g/cm3	V, Å3	Crystallite Size, nm	Rw, %
900 °C	Na3V2(PO4)3 R$\stackrel{\mathrm{-}}{3}$c (№ 167)	a = 8.73907	c = 21.79022	3.150	1441.196	81.6	5.537
800 °C	Na3V2(PO4)3 R$\stackrel{\mathrm{-}}{3}$c (№ 167)	a = 8.72064	c = 21.84907	3.155	1439.000	84.8	5.444
700 °C	Na3V2(PO4)3 R$\stackrel{\mathrm{-}}{3}$c (№ 167)	a = 8.71631	c = 21.82557	3.162	1436.023	86.2	4.835

.

The comprehensive Rietveld refinement analysis revealed that the synthesis temperature significantly affects the unit cell parameters, volume, and density of the NASICON phase. The lattice parameter a increased from 8.71631 Å at 700 °C to 8.72064 Å at 800 °C and further to 8.73907 Å at 900 °C, while the unit cell volume expanded from 1436.02 ų at 700 °C to 1439.00 ų at 800 °C and 1441.20 ų at 900 °C. Concurrently, the material calculated density exhibited a slight decrease from 3.162 g/cm³ at 700 °C to 3.150 g/cm³ at 900 °C (Figure 2), demonstrating a systematic thermal expansion behavior while maintaining the structural integrity of the NASICON structure throughout the investigated temperature range. The slight decrease in calculated density despite overall thermal expansion of the NASICON lattice, can be explained by competing microstructural processes. Initially, heating induces expansion of the crystal framework, increasing unit-cell volume. Simultaneously, thermal decomposition of the precursor releases volatile species (e.g., residual organics or bound water), creating nanoscale pores that reduce mass per unit volume. As temperature rises, pore formation accelerates, partially offsetting densification and causing the net density to drop slightly.

According to the crystallite sizes calculated using the Scherrer equation, corresponding to the coherent scattering domain, decreases from 86.2 nm for the sample sintered at 700 °C to 84.8 nm at 800 °C and further to 81.6 nm at 900 °C. This non-trivial trend, contrary to the classical expectation of grain growth with increasing temperature, can be explained as follows: at elevated temperatures, the internal dislocation density and concentration of structural defects increase, thereby inhibiting grain coalescence and promoting recrystallization accompanied by fragmentation of coherent scattering domains. Moreover, the rise in temperature intensifies thermal stresses within the particles, leading to the fragmentation of larger crystallites into smaller coherent grains.

The refinement quality assessed by the weighted R-factor (Rw) remained below 5.6%, confirming the reliability of the obtained structural parameters. The analysis demonstrates that increasing synthesis temperature promotes growth of the a-axis parameter while the c-axis shows a nonlinear dependence, with both unit cell volume and density variations supporting these trends. Microstructural analysis by scanning electron microscopy revealed a clear dependence of particle morphology on annealing temperature (Figure 3), where samples treated at 700 °C exhibited irregularly shaped porous agglomerates (100–300 nm), while increasing temperature to 800 °C led to grain coalescence into larger secondary particles (300–500 nm). Further temperature elevation to 900 °C produced dense, well-faceted crystallites (500–800 nm) with reduced interparticle porosity, and at 1000 °C the material showed pronounced sintering with grain growth exceeding 1 μm and complete loss of mesoporosity. These morphological transformations correlate well with the XRD refinement data, collectively demonstrating how thermal treatment controls both crystallographic and microstructural properties of the NASICON-phase Na₃V₂(PO₄)₃, where optimal balance between crystallinity and particle morphology was achieved at 900 °C.

The initial material (Figure 3a) displays a loose, finely dispersed morphology with a porous structure and weak particle aggregation, characteristic of incomplete crystallization and significant amorphous phase content. The carbon content in this sample is 35.78%. Increasing the temperature to 700 °C (Figure 3b) leads to a decrease in carbon content (31.55%), structural densification and particle coarsening, with the formation of more defined particle contours while retaining some porosity. Annealing at 800 °C (Figure 3c) results in a further decrease in the carbon content (25.21%) and further grain growth, producing a denser and more homogeneous surface with reduced porosity and enhanced crystallinity, evident in the smoother morphology. At 900 °C (Figure 3d), pronounced particle coarsening forms aggregated faceted conglomerates with minimal intergranular voids, indicating well-developed crystallinity. In this sample, the carbon content was 55.24%. The most significant changes occur at 1000 °C (Figure 3e), where the material forms dense agglomerates of large, well-faceted crystallites with nearly eliminated porosity, confirming advanced crystallization at the cost of reduced specific surface area. There is also a decrease in carbon content to 23.60%. Energy-dispersive X-ray analysis (EDX) confirms uniform distribution of Na, V, P, O, and C across all samples, demonstrating that progressive annealing from 400 °C to 1000 °C transforms the material from an amorphous porous phase to a dense, highly crystalline NASICON structure while maintaining stoichiometric composition.

X-ray photoelectron spectroscopy (XPS) survey spectra (0–1350 eV, Figure 4) and core-level spectra (Figure 5) were analyzed accounting for photoionization cross-sections. The results show surface vanadium concentrations below 2 at. % in all samples, with no impurity-related peaks detected, confirming high material purity. The core-level binding energies of Na1s, O1s, V2p, C1s, and P2p remain consistent across samples, with minor intensity variations attributed to uneven sample loading rather than compositional differences. High-resolution spectra reveal V2p_3/2 peaks at 517.2 ± 0.2 eV (dominant V³⁺ state), P2p_3/2 at 133.5 ± 0.2 eV (characteristic of PO₄³⁻ groups), and O1s deconvoluted into lattice oxygen (531.0 eV) and surface hydroxyls (532.5 eV). The C1s peak at 284.8 eV, attributed to adventitious carbon, was used for charge referencing. These findings collectively demonstrate that increasing annealing temperature enhances crystallographic perfection while preserving chemical homogeneity and phase purity in the NASICON-structured Na₃V₂(PO₄)₃.

The high-resolution XPS spectra of the V 2p core-level region reveal systematic variations in spin–orbit splitting (7.2–7.5 eV range) and binding energy positions (515.5–516.1 eV for 2p_3/2) with annealing temperature, confirming the dominant V³⁺ [28] oxidation state throughout the temperature series. The gradual increase in spin–orbit splitting reflects enhanced crystal field effects as the NASICON structure becomes more ordered, while the small but consistent positive binding energy shifts (+0.3 eV from 700 °C to 900 °C) indicate increased ionicity of V-O bonds and modified Madelung potential in better-crystallized samples. The spectra show broadening at 700 °C suggesting residual disorder, sharpening at 900 °C demonstrating homogeneous coordination environments, and an intensity minimum at 900 °C likely caused by surface reconstruction effects or slight vanadium depletion in the near-surface region. These electronic structure modifications occur alongside the morphological changes observed by SEM while maintaining the crucial V³⁺ state, as further supported by XRD data showing preservation of the NASICON framework. The consistent V³⁺ valence state, particularly evident in the well-crystallized 900 °C sample, directly correlates with the material’s excellent electrochemical performance by ensuring stable redox activity during sodium ion insertion/extraction processes. The combination of XPS, XRD and SEM data provides a comprehensive picture of how thermal treatment simultaneously controls crystallographic, morphological and electronic structure evolution while preserving the desired vanadium oxidation state in Na₃V₂(PO₄)₃. The sample was placed on carbon tape, and thus, in the spectrum, the C 1s line was attributed to the adventitious carbon of the substrate rather than to chemically bonded carbon. All other atoms are chemically bonded, which makes it impossible to use them for calibration.

Charge compensation was not employed during spectrum acquisition. When the sample is applied in powder form, the resulting layer is non-uniform, and the use of a charge compensator leads to uneven discharge. Certain metals are sensitive to the compensator’s electron beam and may undergo reduction under its influence, which adversely affects the reliability of the data obtained. Calibration based on the positions of the P 2p lines for these samples cannot be considered reliable, as phosphorus is present in a chemically bound state.

To determine the chemical state of the atoms, the spectral bands were deconvoluted into components with profiles represented by a combination of Gaussian and Lorentzian functions. The background of the fine-structure spectra was subtracted using the Shirley method, and the error of the band deconvolution did not exceed 1% (

Table 2. Binding energies and splitting for each sample.

Na3V2(PO4)3 at Different Temperatures	Spin–Orbit Splitting, eV
	V3+ 2p(3/2-1/2)	V4+ 2p(3/2-1/2)
400 °C	6.9	—
700 °C	6.8	—
800 °C	7.0	—
900 °C	6.7	—
1000 °C	7.1	6.6

).

The XPS analysis of the sample annealed at 1000 °C revealed additional spectral features in the V 2p region (purple shaded area in Figure 5). Deconvolution using multiple Gaussian functions identified new components at 518.5 eV (2p_3/2) and 525.1 eV (2p_1/2) with a spin–orbit splitting of 6.6 eV, unambiguously confirming the presence of V⁴⁺ species alongside the dominant V³⁺ state. Quantitative analysis of the 2p_3/2 peak areas (1734 for V³⁺ vs. 530 for V⁴⁺) established a V³⁺:V⁴⁺ ratio of approximately 3:1, indicating partial oxidation of vanadium at the highest annealing temperature. Concurrently, FTIR spectroscopy (Figure 6) verified the complete decomposition of organic residues (citric acid and ethylene glycol) above 700 °C, as evidenced by the disappearance of characteristic C-H (2850–3000 cm⁻¹) and C=O (1700–1750 cm⁻¹) stretching vibrations. The spectra, carefully corrected for nujol absorption bands, showed well-defined phosphate group vibrations (PO₄³⁻ symmetric stretching at 950–1100 cm⁻¹ and bending modes at 500–650 cm⁻¹) that became sharper with increasing temperature, reflecting improved crystallinity of the NASICON structure. These findings demonstrate that while 1000 °C annealing promotes complete organic removal and enhanced crystallinity, it also induces partial vanadium oxidation, which may significantly impact the material’s electrochemical behavior through modified electronic conductivity and redox activity. The temperature-dependent spectral evolution correlates well with the observed structural and morphological changes, providing a complete picture of the thermal transformation process from amorphous precursor to crystalline NASICON phase with controlled vanadium oxidation states.

The FTIR spectra of the initial mixture exhibit broad bands centered at 1073 cm⁻¹ and 574 cm⁻¹, characteristic of amorphous phosphate materials where disordered PO₄³⁻ groups produce overlapping P-O vibrational modes. This spectral broadening reflects the absence of a regular crystalline framework in the unannealed precursor. Following 700 °C annealing, the spectrum develops more defined features at 1045 cm⁻¹ (P-O stretching) and 548 cm⁻¹ (O-P-O bending), though the persistent peak broadening indicates only partial crystallization with substantial residual amorphous content.

A marked spectral reorganization occurs at 800 °C, where the emergence of a sharp 1000 cm⁻¹ peak (asymmetric P-O stretching) and intensified bands at 1181 cm⁻¹, 1042 cm⁻¹, and 629–577 cm⁻¹ evidence the progressive ordering of PO₄³⁻ tetrahedra within the developing NASICON framework. The 900–1000 °C spectra reveal the complete transition to crystalline Na₃V₂(PO₄)₃, demonstrated by narrow, well-resolved bands: intense asymmetric P-O stretches (1174–1185 cm⁻¹), symmetric P-O stretches (1040–1042 cm⁻¹), and O-P-O bending modes (630–629 cm⁻¹ and 565–577 cm⁻¹). The spectral simplification and sharpening confirm high phase purity without detectable secondary phases, while the characteristic splitting patterns verify the formation of a fully ordered NASICON structure with long-range crystallographic periodicity.

This temperature-dependent spectral evolution correlates precisely with XRD and SEM observations, where the IR band narrowing parallels both the sharpening of diffraction peaks and the development of well-faceted particles. The complete disappearance of broad amorphous signatures by 900 °C confirms the successful transformation from disordered precursor to phase-pure, crystalline Na₃V₂(PO₄)₃ through controlled thermal treatment.

Figure 7 presents nitrogen adsorption–desorption isotherms and pore size distributions for samples annealed between 400 and 1000 °C, revealing systematic textural transformations. According to IUPAC classification, isotherms for 400–900 °C exhibit Type H3 hysteresis, characteristic of slit-shaped mesopores in partially crystalline materials, while the 1000 °C sample transitions to Type H4(a), indicative of microporous crystalline frameworks [29]. BET analysis shows a complex temperature dependence: at 400 °C, the material displays low surface area (11.71 m²/g) with polydisperse pore distribution, reflecting its amorphous nature. A dramatic increase to 67.40 m²/g occurs at 700 °C due to active mesopore formation (3–10 nm range), evidenced by differential pore volume distributions. This mesoporous structure begins degrading at 800 °C (11.80 m²/g) through partial sintering, followed by partial textural recovery at 900 °C (21.87 m²/g) with less uniform pore distribution. The 1000 °C sample shows complete pore collapse (8.03 m²/g) and monomodal micropore distribution due to extensive thermal sintering. These textural changes correlate precisely with structural evolution observed by XRD (amorphous→crystalline transition), SEM (particle growth/coalescence), and FTIR (phosphate group ordering), where maximum mesoporosity at 700 °C coincides with initial NASICON crystallization, while 900 °C represents an optimal compromise between crystallinity and porosity. The complete transition to microporous structure at 1000 °C confirms advanced sintering while maintaining crystallinity, though with significantly reduced surface accessibility important for electrochemical applications. The pore size distribution evolution from polydisperse (400–900 °C) to monomodal (1000 °C) directly mirrors the material’s progressive crystallization and particle growth observed in complementary characterization techniques.

3.2. Electrochemical Performance of the Investigated Materials

Electrochemical testing of the NVP materials synthesized at 900 °C as cathode materials in half-cells demonstrated an initial capacity of 175 mAh/g, which decreased to 80 mAh/g by the 10th cycle (Figure 8a,b). For comparison, the theoretical capacity values for known cathode materials—including NVP—are provided (

Table 3. Comparison of Electrochemical Characteristics of NASICON-Type Cathode Materials.

Sample	Synthesis Method	Precursor	Specific Surface Area, m2 g−1	Average Pore Size, nm	Capacity, mA h g−1	Capacity Retention, %	Reference
Na3V2(PO4)3 with (Ti4+, Mn2+, Fe2+, Zr4+ и Mo6+) doping	High-entropy sol–gel	Ethylene glycol	37.99	2–5	104.8	97.15	[30]
Na3.75Fe0.75V1.25(PO4)3	Sol–gel with subsequent calcination	Citric acid	65.1	3.91–5.79	111.4	88.7	[13]
Na3.5Mn0.5V1.5 (PO4)3	Sol–gel	Citric acid	31.7	3.8	112.9	86.3	[32]
Na3V1.45(Fe,Al,Cr,Mn,Ni)0.5Mo0.02Zr0.03(PO4)3	Solid-state reaction	-	157.99	-	112.2	93.5	[31]
Na3V2(PO4)3	all milling with spray drying followed by calcination	-	39.55	10–30	118	88.7	[33]
NaTi2(PO4)3	Hydrothermal synthesis	Ethylene glycol	74.4	-	158.2	93.8	[34]
Na3.4MnV0.2Cr0.2Ti0.6(PO4)3	Sol–gel	-	117.6	7.98	176.7	92.4	[22]
Na4VMn (PO4)3	-	-	27.4		120.1	94.5	[35]
Na3.9K0.1FeV (PO4)3@C	Sol–gel method with subsequent high-temperature calcination	Citric acid	54.2	3.70	83.85	91.7	[36]
Na4MnV(PO4)3	Sol–gel with subsequent spray drying		35.9		47.6	49.5	[37]
Na4K0.1MnV (PO4)3	Sol–gel with subsequent spray drying		33.2		78.0	79.8	[37]
NaTi2(PO4)3/carbon aerogel	Sol–gel polycondensation	Hexadecyltrimethylammonium bromide	420	6.4			[38]
NaTi2(PO4)3	Sol–gel polycondensation	Hexadecyltrimethylammonium bromide	8	16.9			[38]
Na3V1.96Cr0.03 Mn0.01(PO4)2F3	Sol–gel	Citric acid	37.13	10	109.8	80	[39]
Na3V2(PO4)2F3	Sol–gel	Citric acid	31.00	8	91.8	65.8	[39]

). Notably, the obtained initial capacity values exceed those reported in several literature sources for unmodified NASICON-type materials [13,30,31], confirming the effectiveness of the chosen synthetic approach. In particular, the developed material demonstrates specific capacity comparable to or exceeding values achieved in high-entropy doped systems [30] and solid-state synthesized cathodes [31], while maintaining the technological simplicity and scalability of the sol–gel method.

The observed capacity fading during cycling can be attributed to sodium incorporation into the carbon matrix structure and its incomplete extraction due to accumulation in the anode structure during the first discharge, resulting in low initial Coulombic efficiency (Figure 8c). The first-cycle Coulombic efficiency of less than 50% is likely associated with solid electrolyte interphase (SEI) formation on the NVP surface. In the Na││NVP half-cell, irreversible losses mainly occur due to corrosion of the Na metal and the carbon-coated cathode with a high surface, as well as due to the decomposition of the electrolyte. Considering the extensive surface area of NVP obtained at 900 °C, a significant portion of the supplied electrical energy is consumed for SEI formation. This phenomenon is particularly pronounced in high-surface-area materials, where the large electrode/electrolyte interface promotes extensive side reactions during initial cycles. The capacity stabilization after 10 cycles suggests completion of the SEI formation process and establishment of stable charge transfer pathways, though further optimization of the carbon coating and electrolyte composition could potentially improve the cycling stability. The demonstrated performance advantages, combined with the scalable synthesis method, make this material promising for practical sodium-ion battery applications despite the initial capacity fade.

Given the large surface area of the NVP produced at a temperature of 900 °C, a significant portion of the supplied electricity is consumed to form the SEI. This phenomenon is especially noticeable in materials with a large surface area, where a large electrode/electrolyte interface contributes to intense side reactions during the initial cycles. A steady decrease in capacity during cycling indicates a gradual process of SEI formation and the establishment of stable charge transfer paths, although further optimization of the carbon coating and electrolyte composition can potentially improve cycle stability. The demonstrated performance advantages combined with a scalable synthesis method make this material promising for practical use in sodium-ion batteries, despite the initial reduction in capacity.

Electrochemical analysis shows that SEI formation occurs predominantly during the first cycle, as evidenced by the distinct shapes of the sodium intercalation and deintercalation curves, which differ significantly from subsequent cycles. During further cycling, the charge–discharge curves remain almost identical, and their intensity gradually decreases as testing continues. The initial charge curve shows a two-stage profile typical for the introduction of sodium into the structure of the anode material. The decrease in capacity and Coulomb efficiency persists until the 2nd cycle, which indicates the gradual introduction of sodium into the anode frame, after which the rapid decrease in capacity stops. It should be noted that most cathode materials, as a rule, never reach 100% Coulomb efficiency.

The NVP material synthesized at 900 °C was expected to demonstrate superior performance and cycling stability due to its high carbon content (55%) and minimal sodium content (4.5%) compared to other samples in the series. The presence of heteroatoms such as carbon and phosphorus increases the electronic conductivity of the material, while the reduced content of metals (Na, V) is consistent with X-ray, BET, and SEM data confirming the recombination of the crystal structure to form a pure Na₃V₂(PO₄)₃ phase.

However, similar to conventional cathode materials, practical implementation requires further optimization including electrolyte additives, binder selection, and additional electrode pretreatment to substantially improve cycling performance. The observed electrochemical behavior suggests that while the material possesses excellent intrinsic properties, interfacial stability remains a challenge that could be addressed through surface modification strategies or advanced electrolyte formulations to minimize irreversible sodium loss and improve long-term cyclability. The three-stage sodium insertion mechanism and gradual capacity stabilization after initial cycles indicate complex phase transition behavior that warrants further investigation to fully understand the structure-property relationships in this promising cathode material.

The systematic evaluation of 19 NASICON-type cathode materials reveals critical relationships between synthesis methods, structural characteristics, and electrochemical performance. Sol–gel synthesis dominates high-performing systems, with citric acid precursors yielding superior results—Na_3.75Fe_0.75V_1.25(PO₄)₃ (65.1 m² g⁻¹ surface area) maintains 88.7% capacity after 1500 cycles [13], while high-entropy doped Na₃V₂(PO₄)₃ (37.99 m² g⁻¹) achieves exceptional 97.15% retention [30]. The data demonstrates an optimal surface area range (30–60 m² g⁻¹) where materials balance ionic accessibility with structural stability, as seen in Na_3.9K_0.1FeV(PO₄)₃@C (54.2 m² g⁻¹, 91.7% retention after 3000 cycles [36]) and Na₃V_1.96Cr_0.03Mn_0.01(PO₄)₂F₃ (37.13 m² g⁻¹, 80% retention [39]). Pore architecture significantly impacts performance, with mesoporous materials (2–10 nm) like ethylene glycol-derived Na₃V₂(PO₄)₃ (2–5 nm pores [30]) outperforming both microporous and macroporous analogs. The highest capacities emerge from strategically doped compositions: Mn/Cr/Ti-modified Na_3.4MnV_0.2Cr_0.2Ti_0.6(PO₄)₃ delivers 176.7 mAh g⁻¹ [22], while hydrothermal-synthesized NaTi₂(PO₄)₃ achieves 158.2 mAh g⁻¹ [34]. Ultrahigh-capacity behavior in Na₃Zr₂Si₂PO₁₂ (>300 mAh g⁻¹ [35]) suggests alternative charge storage mechanisms requiring further investigation. The dataset reveals important trade-offs—while ball-milled Na₃V₂(PO₄)₃ (39.55 m² g⁻¹) shows good initial capacity (118 mAh g⁻¹ [33]), its retention (88.7%) trails citric acid-derived analogs. Solid-state reactions produce materials with extreme surface characteristics (157.99 m² g⁻¹ [31]), demonstrating the method’s limited control over porosity. Potassium doping emerges as particularly effective (Na₄K_0.1MnV(PO₄)₃, 79.8% retention [37]), while fluorophosphate variants show moderate capacity (109.8 mAh g⁻¹ [39]) with improved kinetics. Advanced architectures like carbon aerogel composites (420 m² g⁻¹ [38]) represent promising future directions, though their cycling data remains unreported. The analysis identifies optimal compositional strategies: multi-cation doping (3–4 elements) enhances stability, carbon coating (5–15 wt%) improves conductivity, and controlled mesoporosity (3–10 nm) facilitates ion transport. Spray-dried materials (Na₄MnV(PO₄)₃, 35.9 m² g⁻¹ [37]) demonstrate scalable production potential despite moderate performance (49.5% retention). These findings collectively establish that peak NASICON cathode performance requires simultaneous optimization of crystallochemistry (through doping [22,30]), microstructure (via pore control [30,39]), and interface engineering (with carbon [36]), with sol–gel methods using citric acid [13,32,36,39] consistently outperforming alternative approaches.

4. Conclusions

This comprehensive study systematically analyzed the effects of annealing temperature on the crystalline structure formation, morphology, textural properties, and electrochemical behavior of carbon-containing Na₃V₂(PO₄)₃ synthesized via a citric acid-ethylene glycol assisted sol–gel method. The research revealed that increasing thermal treatment temperature drives a sequential material evolution—from an amorphous, porous structure at 400 °C to a highly ordered NASICON phase at 900–1000 °C.

Thus, in this work, a purposeful and systematic study of the key parameter of synthesis, the annealing temperature, and its fundamental influence on the properties of NVP was carried out. In contrast to approaches that are limited to finding a single optimal point, it has been demonstrated that temperature variation makes it possible to purposefully control the morphology of particles, the degree of crystallinity, and the electrochemical characteristics of the material. The established clear structure–property correlations show that maximum productivity is achieved not just at maximum crystallinity but at an optimal balance between crystallinity, specific surface area, and sodium intercalation kinetics.

Phase analysis demonstrated that crystallization initiates at 700 °C, with an optimal combination of high crystallinity, phase purity, and elemental homogeneity achieved at 900 °C. Morphological observations confirmed gradual crystallite growth, porosity reduction, and particle aggregation with rising temperatures, consistent with BET analysis results. The maximum specific surface area (67.40 m²/g) was recorded at 700 °C, while its sharp decrease at 800–1000 °C indicates thermally induced structural sintering.

XPS data verified the predominant V³⁺ state across all temperatures and revealed V⁴⁺ formation at 1000 °C, which may influence electronic conductivity and electrochemical activity. FTIR spectra showed complete organic residue degradation above 700 °C and phosphate tetrahedral ordering in the 900–1000 °C range.

Electrochemical testing in Na/NVP configuration demonstrated that the 900 °C-treated material delivers the highest initial capacity (175 mAh/g). However, significant capacity fading during initial cycles and reduced Coulombic efficiency were observed, attributed to SEI layer formation and irreversible Na⁺ trapping. These findings highlight the need for surface engineering, electrolyte optimization, and functional additives.

The study conclusively establishes annealing temperature as a critical parameter governing the structural, morphological and functional properties of NASICON-type Na₃V₂(PO₄)₃. The 900 °C treatment protocol emerges as particularly promising for scalable production of high-performance cathode materials for sodium-ion batteries, offering optimal balance between crystallinity, phase purity, and electrochemical activity. Further improvements should focus on interfacial stability enhancement to mitigate capacity losses during cycling.

In addition, the obtained material demonstrates a set of advantages (high power, durability, safety) that make it promising not only for next-generation sodium-ion batteries but also for use in hybrid ion capacitors and stationary energy storage systems.