Purification of high-performance vanadium electrolyte from Al-containing solutions via NH4Al(SO4)2·12H2O crystallization and saponified P204 (D2EHPA) extraction

Hong Liu; Tao Liu; Yimin Zhang; Hongru Qu; Youwen Hu

doi:10.25259/AJC_881_2025

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Original Article

:19;

8812025

doi:

10.25259/AJC_881_2025

Purification of high-performance vanadium electrolyte from Al-containing solutions via NH₄Al(SO₄)₂·12H₂O crystallization and saponified P204 (D2EHPA) extraction

Hong Liu^a,b,c,d,*, Tao Liu^a,b,c,d,*, Yimin Zhang^a,b,c,d, Hongru Qu^a,b,c,d, Youwen Hu^a,b,c,d

aSchool of Resource and Environmental Engineering, Wuhan University of Science and Technology, Wuhan, China

bState Environmental Protection Key Laboratory of Mineral Metallurgical Resources Utilization and Pollution Control, Wuhan University of Science and Technology, Wuhan, China

cHubei Collaborative Innovation Center for High Efficient Utilization of Vanadium Resources, Wuhan University of Science and Technology, Wuhan, China

dHubei Provincial Engineering Technology Research Center of High Efficient Cleaning Utilization for Shale Vanadium Resource, Wuhan University of Science and Technology, Wuhan, China

*Corresponding authors: E-mail addresses: liuhong126@wust.edu.cn (H. Liu), tkliutao@126.com (T. Liu)

Received: 2025-07-27, Accepted: 2025-09-02, Epub ahead of print: 2025-11-10, Published: 2026-01-05

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Abstract

Traditional vanadium electrolyte production faces challenges of complex processes, high costs, and inefficient impurity removal. This study developed an integrated crystallization-solvent extraction method to directly produce high-performance electrolyte from high-aluminum vanadium shale leachate (Al: 21.2 g/L, V: 54.2 g/L). Through ammonium alum crystallization under optimal conditions (NH₄⁺/Al³⁺=1:1, 5°C, 6 h), 51.67% aluminum was removed with <1% vanadium loss. Subsequent five-stage countercurrent extraction achieved 96.04% vanadium recovery, while four-stage stripping produced a 2.0 mol/L V electrolyte. The obtained electrolyte exhibited excellent properties: viscosity of 5.00 mm²/s, conductivity of 253.7 mS/cm, and electrochemical performance comparable to commercial standards. Battery tests demonstrated outstanding stability with average coulombic efficiency of 87.86%, voltage efficiency of 93.00%, and energy efficiency of 81.70% over 80 cycles at 40 mA/cm². This short process offers strong potential for industrial application in vanadium redox flow batteries.

Keywords

Black shale

Electrochemical performance

Electrolyte

Vanadium-rich solution with high Al concentration

Solvent extraction

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1. Introduction

As fossil fuel reserves diminish and environmental regulations tighten, renewable energy sources such as solar and wind have gained widespread adoption [1-3]. However, their inherent intermittency necessitates reliable energy storage to stabilize output [4-5]. Among large-scale electrochemical storage options, the Vanadium Redox Flow Battery (VRFB) is the most mature and commercially advanced, known for its high safety, excellent reaction kinetics, fast response, long cycle life, and high efficiency [6-9].

Nevertheless, the high cost of vanadium electrolytes remains a major barrier to broader VRFB application [10]. While VOSO₄ dissolves favorably in sulfuric acid, its expense has led to the adoption of more economical V₂O₅ as a precursor [11]. However, high-purity V₂O₅ (>99.5%) is still required to avoid detrimental impurities, keeping preparation costs substantial [12-14]. Vanadium shale, which contains low levels of harmful elements such as manganese and chromium, presents a promising raw material for direct electrolyte production [15]. Using it to prepare electrolytes through purification and enrichment could simplify manufacturing, reduce environmental impact, and support the advancement of VRFB technology [16].

Liquid-liquid extraction has emerged as an effective and selective method extensively utilized for the concentration and refinement of vanadium-bearing solutions [17-19]. Organic phosphonic acid-based extractants are preferentially employed in tetravalent vanadium refinement processes owing to their outstanding separation performance and selectivity [10,20]. In contrast, amine extractants are predominantly employed for the recovery and fortification of high-valency vanadium (V⁵⁺) [21,22]. In the acid leaching solution derived from vanadium shale, vanadium predominantly exists in the tetravalent state. Consequently, phosphonic acid extractants serve as a key extraction system for directly preparing electrolytes from vanadium shale, owing to their suitability for tetravalent vanadium recovery. The investigation conducted by Zhang et al. involved the application of D2EHPA for solvent extraction of vanadium from converter slag acid leachates [12]. Extraction was conducted at a pH of 1.8, where EDTA was used to form a complex with Fe(III), thereby enabling efficient separation of V and Fe. Implementation of a sequential extraction process yielded a vanadium electrolyte meeting high purity standards. In their investigation of V(IV) purification, Li et al. employed a dual-extractant methodology utilizing Ethylhexylphosphoric acid mono-2-ethylhexyl ester in combination with Tributyl phosphate (EHEHPA) in combination with TBP to separate iron and aluminum impurities [23]. At pH 1.8, five-stage processing generated VOSO₄ electrolyte (76.5 g/L V(IV)) meeting industrial standards. Nevertheless, this multi-step procedure proved inefficient, requiring excessive processing time while demonstrating suboptimal vanadium recovery rates. In our previous study [24], the water leaching solution from sodiumized vanadium shale was used as the feedstock, with D2EHPA serving as the extractant. Within an alkaline chloride-rich system set at pH 2.6, researchers produced a 1.77 M VOSO₄ electrolyte solution. While significant progress has been made in the purification and enrichment of vanadium from solutions with high iron content, the treatment of high-aluminum vanadium solutions continues to pose considerable challenges. In particular, the incomplete removal of aluminum impurities often leads to deteriorated electrolyte performance, limiting the practical application of such resources. To address this issue, the present study proposes a novel and integrated approach combining ammonium alum crystallization with solvent extraction. This strategy not only achieves efficient separation of vanadium from aluminum, the primary impurity, but also enables the direct production of high-quality vanadium electrolyte, while simultaneously recovering ammonium alum as a valuable by-product.

The research was conducted through a systematic investigation: first, the impact of aluminum ions on solvent extraction efficiency and electrolyte performance was thoroughly evaluated. Subsequently, an optimized crystallization process was applied to effectively remove aluminum impurities from the vanadium-rich solution. The resulting purified and vanadium-enriched solution then underwent methodical optimization of extraction and stripping conditions. Finally, the comprehensively prepared electrolyte was rigorously characterized for its physical, chemical, and electrochemical properties, and compared in detail with a standard electrolyte. This work provides an efficient and technically feasible pathway for the sustainable production of high-performance vanadium electrolytes from aluminum-rich sources, offering significant practical implications for the advancement of vanadium redox flow batteries.

2. Materials and Methods

2.1. Reagents and raw materials

This study employed a vanadium-bearing solution sourced from a Shaanxi-based vanadium processing facility as the precursor for VOSO₄ electrolyte synthesis. The principal chemical constituents of the feedstock solution have been detailed in Table 1. The listed elements represent the major ionic components of the industrial leaching solution used in this study, as determined by inductively coupled plasma-optical emission spectroscopy (ICP-OES) and ion chromatography.

Table 1. The main chemical components of the vanadium-rich solution (mg/L).

Elements	SO₄^2-	V	Al	Fe	Na	K	Mg
Concentration	269910	54234.90	21211.30	2782.28	707.58	545.69	3304.12

Acid stripping with sulfuric acid generated a vanadium-rich solution with remarkably low pH (0.27), along with a sulfate concentration of 269.91 g/L and a vanadium concentration of 54.23 g/L, as presented in Table 1. The impurities are primarily aluminum and iron, with concentrations of 21211.30 mg/L and 2782.28 mg/L, respectively. Other impurities include sodium at 707.58 mg/L, potassium at 545.69 mg/L, and magnesium at 3304.12 mg/L. This vanadium-rich solution belongs to the sulfuric acid system and is characterized by high acidity, high vanadium concentration, and the presence of multiple impurities.

All major chemicals, including (NH₄)₂SO₄, Na₂SO₃, H₂SO₄, NaOH, D2EHPA (D2EHPA, >95%), TBP (>98.5%), and sulfonated kerosene, were of analytical or industrial grade and were obtained from Sinopharm Chemical Reagent Co., Ltd.

2.2. Methods

2.2.1. Crystallization process

A 100 mL beaker was charged with 50 mL of the vanadium-containing solution, after which a calculated quantity of (NH₄)₂SO₄ was introduced under continuous agitation. The mixture was subsequently placed in a constant-temperature cooling apparatus maintained at a crystallization temperature of 5°C. The crystallization process was carried out in static conditions without additional stirring. After a specified duration of crystallization, solid-liquid separation was performed to obtain the aluminum-depleted solution and ammonium aluminum sulfate crystals.

2.2.2. Liquid-liquid extraction experiment

Extraction trials were carried out in 100 mL beakers under controlled temperature conditions (25°C) with stirring set to 250 revolutions per minute. The organic phase composition included D2EHPA, TBP, and sulfonated kerosene with a volume ratio of 20%:5%:75%. The schematic for the synthesis of vanadium electrolyte in this study has been shown in FigureFigure 1.

The schematic for the synthesis of the vanadium electrolyte in this study.

2.2.2. Electrochemical measurements and charge–discharge tests

The electrochemical characterization employed a standard three-electrode configuration, utilizing a 1 cm² graphite working electrode, 4 cm² platinum auxiliary electrode, and saturated calomel reference electrode. Cyclic voltammetry (CV), electron impedance spectroscopy (EIS), and galvanostatic charge-discharge measurements were performed following established protocols from our prior research [20].

2.2.3. Instruments and analysis

pH values were recorded using a PHS-3C meter (Shanghai INESA), and solution chemistry was characterized by inductively coupled plasma-atomic emission spectroscopy (ICP-AES). The method for calculating the crystallization efficiency is shown as Eq.(1):

(1)

Crystallizationefficiency = \frac{C_{0} V_{0} - C_{1} V_{1}}{C_{0} V_{0}} \times 100 %

C₀ (mg/L) and C₁ (mg/L) refer to the ion concentrations before and after purification, whereas V₀ (L) and V₁ (L) indicate the volumes of the untreated and processed solutions.

The main indicators and calculation methods in solvent extraction are shown in Eqs. (2-4):

(2)

D = \frac{C_{o r g}}{C_{a q}}

(3)

E xtraction p e r c e n t a g e = \frac{D}{D + (V_{aq} / V_{org})} \times 100 %

(4)

β_{a, b} = \frac{D_{a}}{D_{b}}

where C(feed solution) and C_org (organic phase) are the ion concentrations, D is the distribution coefficient (organic-to-aqueous phase ratio), β_a/b is the vanadium/impurity separation factor, and Vand V_org are the respective volumes of the aqueous and organic phases.

3. Results and Discussion

3.1. Regulation of Al impurities in vanadium-rich solution

3.1.1. Effect of Al concentration on vanadium extraction

The vanadium-enriched solution is contaminated with considerable aluminum impurities. Understanding aluminum’s impact on vanadium recovery under high-concentration conditions is crucial for optimizing the downstream separation process. Figure 2 displays the vanadium and aluminum extraction rates from V-rich solutions at varying Al concentrations.

Effect of aluminum concentration on vanadium and aluminum extraction in the vanadium-rich solution (Saponification degree 60%, D2EHPA concentration 40%, initial pH 1.0, phase ratio (O/A) 1:1, extraction time 8 min).

As shown in Figure 2, the presence of aluminum significantly affects vanadium extraction. In pure vanadium solutions, the vanadium extraction efficiency reached 83.41%. When the aluminum ion concentration increased to 0.1 mol/L, the vanadium extraction rate gradually decreased to 81.45%. At an aluminum ion concentration of 0.2 mol/L, the vanadium extraction rate dropped sharply to 70.71%, and further increases in aluminum ion concentration led to a continued but slower decline in vanadium extraction efficiency. The results presented in Figure 2 indicate that the co-extraction of aluminum impurity ions reduces the utilization efficiency of the extractant and inhibits vanadium extraction. Excessive aluminum impurity concentrations are detrimental to efficient vanadium extraction.

In this study, the aluminum concentration in the vanadium-enriched solution exceeded 20 g/L. During the extraction process, aluminum not only co-extracts with vanadium, thereby affecting its extraction efficiency, but also potentially leads to emulsification and extractant poisoning. Therefore, pre-removal of aluminum impurities prior to extraction was considered as a strategy to mitigate these adverse effects.

3.1.2. Effect of Al concentration on vanadium electrolyte performance

To delineate the influence scope of aluminum impurities on the performance of vanadium electrolyte and provide theoretical guidance for the removal of aluminum impurities during the electrolyte preparation process, this study systematically examined the effects of different concentrations of aluminum impurities on the physicochemical properties of vanadium electrolyte.

Experimental results in Figure 3 indicate that increasing Al³⁺ concentration in the electrolyte produces two effects: conductivity suffers a significant reduction while viscosity shows consistent enhancement. Specifically, in terms of viscosity, when the aluminum ion concentration rises from 0 g/L to 2 g/L, the viscosity increases slowly; when the concentration further elevates to 5 g/L, the growth trend of viscosity becomes more pronounced, with an overall increase of 5.9%. Regarding conductivity, as the concentration of aluminum impurity ions increases from 0 g/L to 5 g/L, the conductivity demonstrates a slow downward trend, with an overall decrease of 16.7%. The elevated Al³⁺ concentration in the electrolyte raises internal resistance, while increased viscosity impedes V-ion diffusion. These combined effects amplify concentration polarization, diminish electrode reaction reversibility, and ultimately compromise vanadium battery performance.

Effect of Al3+ concentration on the viscosity and conductivity of vanadium electrolyte.

The CV method represents a standard electrochemical testing procedure that measures both the active behavior and reversible properties of electrolyte systems [25]. Figure 4 depicts the CV curves at different aluminum impurity concentrations, and Table 2 lists the key parameters calculated based on the CV curves. It can be inferred from Figure 4 and Table 2 that after introducing aluminum impurity ions of varying concentrations into the electrolyte, the anodic peak current remains approximately at 70 mA, the cathodic peak current density is approximately -45 mA, and the peak current density ratio remains at around 1.5, indicating that the aluminum impurity ions have a relatively minor impact on the peak current density. Nevertheless, as the aluminum ion concentration increases from 0 g/L to 1 g/L, the peak potential difference remains essentially stable at approximately 0.4 V; but when the aluminum ion concentration increases to 2 g/L, the peak potential difference significantly increases to 1.57 V. This phenomenon suggests that the reversibility of the electrode reaction decreases with the increase in aluminum ion concentration. The possible reason lies in the continuous increase in electrolyte viscosity, which leads to a decrease in the diffusion coefficient of vanadium ions, thereby increasing the resistance that the electrode reaction process needs to overcome and reducing the reversibility of the reaction. The experimental findings indicate that maintaining aluminum impurity levels under 1500 mg/L is critical for the proper functioning of vanadium-based electrolytes.

CV curves of electrolytes with different Al3+ concentrations.

Table 2. CV test parameters of electrolytes with different Al³⁺ concentrations.

Al(mg/L)	Anodic peak		Cathodic peak		△E_p (V)	i_pa/i_pc
Al(mg/L)	i_pa (mA/cm²)	E_pa (V)	i_pc (mA/cm²)	E_pc (V)	△E_p (V)	i_pa/i_pc
0	68.541	1.258	-48.889	0.841	0.418	1.40
50	69.673	1.259	-48.768	0831	0.428	1.43
500	70.179	1.258	-49.012	0.829	0.429	1.44
1000	71.395	1.251	-46.819	0.798	0.453	1.52
2000	71.532	1.312	-45.776	0.782	0.530	1.57
5000	67.496	1.303	-45.745	0.776	0.527	1.48

3.1.3. Removal of Al impurity ions by NH₄Al(SO₄)₂·12H₂O crystallization

Currently, the primary methods for aluminum ion removal include precipitation, crystallization, extraction, and adsorption [26,27]. In strongly acidic sulfate media, aluminum readily forms the stable double salt ammonium alum when ammonium ions are introduced. This reaction is highly selective for aluminum over vanadium under controlled conditions. In this study, the low-temperature crystallization method was utilized to remove aluminum ions [28]. By adding ammonium sulfate to the vanadium-rich solution, ammonium ions reacted with aluminum ions to form ammonium alum crystals, which subsequently crystallized and precipitated. The main reaction is shown in reaction (5).

(5)

{12H}_{2} O + {Al}^{3 +} + {2SO}_{4}^{2 -} + {NH}_{4}^{+} \to ({NH}_{4}) Al {({SO}_{4})}_{2} \cdot {12H}_{2} O

In this study, ammonium sulfate was chosen as the ammonium source. Based on the solubility of ammonium alum at various temperatures and energy consumption considerations, an experimental temperature of 5°C was determined. The effects of standing time and ammonium sulfate dosage on aluminum removal were investigated, and the results have been displayed in Figure 5.

(a) Effects of crystallization time and (b) the amount of added ammonium sulfate on aluminum removal.

As shown in Figure 5(a), when the molar ratio of NH₄⁺ to Al³⁺ is 1:1, the aluminum removal rate increases gradually over time, with the rate of increase slowing after 4 h. This indicates that the reaction progresses relatively quickly. Balancing removal efficiency and treatment effectiveness, a crystallization time of 6 h was selected. As shown in Figure 5(b), under the condition of 6 h, increasing the molar ratio of NH₄⁺ to Al³⁺ to 1:1 achieves an aluminum removal rate of 51.67%. Further increasing the ammonium sulfate dosage to a molar ratio of 3:1 only raises the removal rate to 69.33%. Given the potential for excessive ammonium sulfate addition to increase reagent consumption and filtration pressure, a molar ratio of NH₄⁺ to Al³⁺ of 1:1 was deemed optimal.

For the generated white crystals, after filtration, they were washed repeatedly with deionized water until the washing solution became clear and colorless, aiming to recover the vanadium adhered to the surface and prevent unnecessary vanadium loss caused by phase entrainment. Subsequently, XRD analysis was performed on the washed product to confirm its phase composition. The analysis results have been depicted in Figure 6. As evidenced by Figure 6, through comparison with the standard X-ray diffraction (XRD) patterns, it can be ascertained that the product is NH₄Al(SO₄)₂·12H₂O, and no impurity peaks were detected. The diffraction pattern of the crystallized product perfectly matched the reference pattern for NH₄Al(SO₄)₂·12H₂O, and the sharp, intense peaks indicate high crystallinity. The absence of any extra diffraction peaks corresponding to vanadium compounds confirms that vanadium is not incorporated into the crystal lattice. This demonstrates the structural purity of the alum product with respect to vanadium.

XRD patterns and crystal images of ammonium-cooled crystallization of vanadium-rich solution.

Scanning electron microscopy-energy dispersive X-ray (SEM-EDS) analysis was carried out on the ammonium alum crystals, and the analysis and test results are shown in Figure 7. As revealed in Figure 7, the ammonium alum crystals formed a regular octahedral crystal structure. In addition. elemental mapping (Figure 7) showed a homogeneous distribution of Al, S, O, and N, while V was only faintly detected on the crystal surfaces, attributed to adsorbed solution (entrainment), not lattice incorporation. This further validates that aluminum ions are crystallized and removed in the form of ammonium alum, and the loss of vanadium mainly remains in trace amounts on the crystal surface.

SEM-EDS analysis of the crystalline product of ammonium alum.

The well-defined octahedral morphology, a signature of pure, crystalline ammonium alum, is both an indicator of and a contributor to the high efficiency and selectivity of aluminum removal. It signifies high purity and selectivity by reflecting controlled crystallization favoring a single alum phase without impurity-encapsulating mixed or amorphous precipitates; enables efficient solid-liquid separation due to large, equidimensional crystals’ good settling velocities and porous filter cakes, reducing solution and vanadium entrapment compared to finer or amorphous forms; and facilitates effective washing, as regular surfaces and good packing let wash water penetrate easily to remove entrained vanadium-rich solution, keeping vanadium loss below 1%.

In summary, by adding ammonium sulfate to generate ammonium alum, impurity aluminum can be effectively removed. Additionally, as a by-product, ammonium alum can be extensively applied in areas such as water purifying agents, mordants, paper sizing agents, and can also be utilized in medicine, baking powder, leather tanning, and food additives.

3.2. Preparation of vanadium electrolyte by saponified D2EHPA

3.2.1. Selection of solvent extraction systems

To investigate the influence of various extraction systems on the separation of vanadium and the separation of impurities (iron and aluminum), a vanadium-rich solution derived from shale was utilized as the raw material. Experiments were carried out under these specified conditions: sulfonated kerosene as the diluent, sodium hydroxide as the saponifying agent, a saponification degree of 60%, an initial pH of 2.0, an oil-to-aqueous phase ratio of 2:1 (O/A), an extraction time of 8 min, and a temperature of 25°C. Visual representation of the experimental outcomes appears in Figure 8, with corresponding quantitative data presented in Table 3.

Effect of different extraction systems on vanadium extraction.

Table 3. The extraction rates and separation coefficients of iron and aluminum in different extraction systems.

Extraction systems	E(Fe)(%)	E(Al)(%)	β_V/Fe	β_V/Al
40%D2EHPA (Non-saponification)	22.62	12.88	5.03	9.95
40%D2EHPA (Saponification)	45.81	17.43	4.82	19.31
40%EHEHPA (Non-saponification)	18.24	6.20	3.78	12.77
40%EHEHPA (Saponification)	38.95	8.70	2.76	18.48
30%D2EHPA +10%EHEHPA (Non-saponification)	32.74	15.50	2.99	7.95
30%D2EHPA+10%EHEHPA (Saponification)	50.91	19.37	4.56	19.71

As shown in Figure 8, in a conventional non-saponified extraction system, the vanadium extraction rates for single D2EHPA-sulfonated kerosene and single EHEHPA-sulfonated kerosene were 59.54% and 42.77%, respectively. D2EHPA demonstrated superior extraction capacity and better performance at low pH compared to EHEHPA. Subsequently, with D2EHPA as the primary extractant, the addition of EHEHPA, TBP, C272, and N235 was investigated to determine whether they could enhance the synergistic extraction effect and improve the vanadium extraction rate. However, the results indicated that the inclusion of these additional extractants did not lead to any significant improvement. In contrast, in the saponified system, the vanadium extraction rates of all extraction systems increased substantially. Notably, the highest extraction rates were achieved by saponified D2EHPA + EHEHPA (82.56%) and saponified D2EHPA (80.30%). These findings clearly indicate that organic phase saponification prior to extraction leads to significantly improved vanadium separation from the rich solution.

The extraction and separation efficiencies of the main impurities (iron and aluminum) using D2EHPA, EHEHPA, and D2EHPA + EHEHPA extraction systems under both saponified and non-saponified conditions have been detailed in Table 3. The results indicate that acidic phosphine extractants exhibit significantly better separation performance for aluminum than for iron, suggesting that the separation of vanadium and iron is more challenging. After saponification, although the vanadium extraction rate increased markedly, the extraction rate of iron also rose, thereby weakening the separation effect between vanadium and iron. The separation coefficients β_V/Fe were ranked as follows: D2EHPA > D2EHPA + EHEHPA > EHEHPA. Conversely, the extraction rate of aluminum did not increase significantly, indicating a more effective separation. The separation coefficients β_V/Fe were ranked as: D2EHPA + EHEHPA > D2EHPA > EHEHPA. Considering all factors comprehensively, saponified D2EHPA was selected as the optimal extraction system for this study due to its high vanadium extraction rate, favorable separation efficiency of vanadium from impurities, and cost-effectiveness. In the saponified D2EHPA solvent extraction process, the solution remained highly acidic. In such media, the extraction of metal cations by acidic extractants like D2EHPA is favored over the extraction of ammonium ions (NH₄⁺), which are very weakly extracted under strong acid conditions.

3.2.2. Extraction

Based on the aforementioned research findings, ammonium sulfate was added to achieve a molar ratio of ammonium ions to aluminum ions of 1:1 under conditions of 5°C and a standing time of 6 h. This process effectively pre-removed impurity aluminum via ammonium alum crystallization. Subsequently, sodium sulfite was introduced as a reducing agent at a m(Na₂SO₃):m(Fe) ratio of 2:1, with a reduction temperature of 50°C and stirring for 30 min. This step aimed to reduce iron ions in the feed solution and suppress their oxidation during the extraction process. The main composition of the pre-treated rich vanadium solution after these operations has been summarized in Table 4. As shown in Table 4, the vanadium concentration in the treated rich vanadium solution increased from 54,234 mg/L to 56,721 mg/L, while the aluminum concentration decreased from 21,211 mg/L to 10,313 mg/L. The treated rich vanadium solution was then utilized as the extraction stock solution, and further optimization of the preparation process parameters enabled effective impurity regulation and successful preparation of the vanadium electrolyte.

Table 4. Main composition of vanadium-rich solution after pretreatment (mg/L)

Elements	V	Fe	Al	Zn	Na
Feed solution	56721.21	2776.54	10313.06	3871.25	711.19

A comprehensive study was conducted to investigate the influence of saponification degree, initial pH of the feed solution, organic-to-aqueous phase ratio (O/A), and extraction duration on the selective recovery of vanadium relative to key impurities. The findings have been presented in Figure 9. With extraction parameters set at a pH of 1.0, an 8-min contact time, an extractant concentration of 30 vol%, and an O/A ratio of 3:1, the effect of extractant saponification on the selective extraction efficiencies for vanadium, iron, and aluminum was assessed (Figure 9a). Figure 9(a) demonstrates that as saponification increased from 40% to 80%, both vanadium and iron extraction efficiencies exhibited a steady rise; however, aluminum recovery experienced only marginal enhancement. This indicates that elevated levels of saponification favor vanadium extraction significantly, as evidenced by an increase in the separation factor between vanadium and aluminum (β_V/Al) from 5.9 to 24.9, reflecting improved V/Al selectivity. At 60% saponification, a high vanadium extraction rate (about 85%) was achieved while still maintaining acceptable selectivity over iron. Further increasing saponification provided diminishing returns in V extraction but significantly worsened iron co-extraction, making the subsequent purification more difficult. Fully saponified D2EHPA would maximize V extraction but at the severe cost of poor Fe/V selectivity and potential operational issues, which is undesirable. As a result, it was determined that a saponification degree of 60% represents the optimal level for achieving efficient separation.

The influence of extraction conditions (a) saponification degree, (b): initial pH, (c) extraction time, (d) extractant concentration, (e) phase ratio, countercurrent extraction stage on the separation effect of vanadium and impurities.

Under the same extraction conditions with a saponification degree of 60%, the effect of initial pH on the extraction and separation of vanadium, iron, and aluminum was examined, as illustrated in Figure 9(b). From Figure 9(b), it can be seen that as the initial pH increases from 0.0 to 2.0, the extraction rate of vanadium initially rises rapidly and then stabilizes. In contrast, the extraction rates of iron and aluminum show a continuous upward trend, with the increase in iron being more pronounced than that of aluminum. Although increasing the initial pH enhances the extraction of vanadium, it also leads to higher co-extraction of impurities. A lower initial pH thus enhances the selective recovery of vanadium over impurities while minimizing alkali usage and preventing vanadium loss during pH regulation. Considering the separation factors between vanadium-iron (β_V/Fe) and vanadium-aluminum (β_V/Al), the optimum initial pH was determined to be 1.0.

The effects of extraction duration on vanadium, iron, and aluminum separation were examined using fixed parameters: 60% saponification, pH 1.0, 30 vol% extractant concentration, and a 3:1 organic-to-aqueous phase ratio (O/A) (Figure 9c). As depicted in Figure 9(c), vanadium extraction efficiency initially rose with time, peaking at 8 min before declining, whereas extraction times beyond this point led to reduced recovery. In contrast, the extraction rate of iron continues to rise, while the extraction rate of aluminum remains relatively stable. Prolonged extraction time may lead to the oxidation of iron ions, intensifying competitive extraction and adversely affecting the separation of vanadium. Considering the changes in the separation coefficients of vanadium and iron (β_V/Fe) and vanadium and aluminum (β_V/Al), an extraction time of 8 min was determined to be optimal.

Under the extraction conditions of a saponification degree of 60%, an initial pH of 1.0, an extraction time of 8 min, and a phase ratio (O/A) of 3:1, the influence of extractant concentration on the extraction and separation of vanadium, iron, and aluminum was studied, as depicted in Figure 9(d). From Figure 9(d), it can be observed that as the extractant concentration increases from 10 vol% to 50 vol%, the extraction rates of vanadium and iron exhibit a continuous upward trend, while the extraction rate of aluminum shows a slight increase. Notably, the increase in the extraction rate of vanadium slows down when the extractant concentration exceeds 30 vol%, whereas the extraction rate of iron continues to rise. The separation coefficient of vanadium and iron (β_V/Fe) reaches its maximum at an extractant concentration of 30 vol%. Consequently, 30 vol% was determined to be the most effective extractant concentration.

Under the extraction conditions of a saponification degree of 60%, an initial pH of 1.0, an extraction time of 8 min, and an extractant concentration of 30 vol%, the influence of the phase ratio (O/A) on the extraction and separation of vanadium, iron, and aluminum was studied, as shown in Figure 9(e). From Figure 9(e), it is clear that as the phase ratio (O/A) increases from 1:1 to 4:1, the extraction rate of vanadium exhibits a continuous upward trend. Experimental results demonstrated that vanadium extraction efficiency substantially improved from 30.15% to 82.56% as the phase ratio (O/A) was elevated from 1:1 to 3:1. During this process, iron recovery increased from 19.01% to 31.76%, with aluminum showing a similar trend from 14.53% to 25.75%. Beyond the 3:1 ratio, vanadium extraction exhibited diminishing returns, whereas iron and aluminum extraction persisted in their upward trajectory. Considering both impurity suppression and economic factors, the optimal phase ratio was established at 3:1.

3.2.3. Stripping

The effect of sulfuric acid concentration and stripping time on V stripping was studied in this section, and the results are illustrated in Figure 10. Under the stripping conditions of a sulfuric acid concentration of 4 mol/L and a stripping time of 20 min, the influence of the stripping phase ratio (O/A) on vanadium stripping was evaluated. As shown in Figure 10(a), as the stripping phase ratio (O/A) increased from 4:1 to 12:1, the vanadium stripping decreased from 88.98% to 47.9%. Within the range of 6:1 to 8:1, the vanadium extraction efficiency dropped significantly from 83.85% to 63.28%. To prepare a 2 mol/L vanadyl sulfate solution, a sixfold enrichment was required based on calculations. Considering the comprehensive performance of vanadium stripping, a stripping phase ratio (O/A) of 6:1 was selected.

The influence of stripping conditions ((a) phase ratio, (b) H2SO4 concentration, (c) stripping time) on vanadium separation.

Subsequently, under the same stripping conditions with a phase ratio (O/A) of 6:1 and a stripping time of 20 min, the impact of sulfuric acid concentration on vanadium stripping was analyzed. As depicted in Figure 10(b), when the sulfuric acid concentration increased from 2 mol/L to 5 mol/L, the vanadium stripping efficiency rose from 55.29% to 91.84%. Notably, within the range of 3 mol/L to 4 mol/L, the vanadium extraction efficiency increased rapidly from 58.17% to 83.85%, indicating that higher sulfuric acid concentrations promote vanadium stripping. Based on these findings, a sulfuric acid concentration of 4 mol/L was chosen as the optimal condition.

Finally, under the stripping conditions of a phase ratio (O/A) of 6:1 and a sulfuric acid concentration of 4 mol/L, the effect of stripping time on vanadium stripping was examined. As shown in Figure 10(c), as the stripping time increased from 10 min to 40 min, the vanadium stripping efficiency improved from 58.71% to 84.28%. Within the first 10 to 20 min, the vanadium extraction efficiency increased markedly from 58.71% to 82.12%, and beyond 20 min, the stripping rate stabilized. Therefore, a stripping time of 20 min was determined to be the optimal condition.

3.2.4. Preparation of vanadium electrolyte through continuous countercurrent extraction

Through methodical evaluation of preceding single-variable tests, the most efficient extraction parameters were established. Specifically, the saponification degree was set to 60%, the initial pH of the feed solution was adjusted to 1.0, the volume ratio of D2EHPA to sulfonated kerosene was 30%, the phase ratio (O/A) was 3:1, and the extraction time was 8 min. Under these conditions, the vanadium extraction efficiency in a single-stage countercurrent process reached 82.36%, while the extraction efficiencies of impurity elements iron and aluminum were 29.76%, and 24.75%, respectively. The optimal stripping conditions were identified as follows: a phase ratio (O/A) of 6:1, a stripping agent concentration of 4 mol/L sulfuric acid, and a stripping time of 20 min. Under these conditions, the single-stage vanadium stripping efficiency reached 82.12%.

To further enhance the recovery of vanadium and achieve effective separation of impurities, multi-stage countercurrent extraction and stripping processes were implemented. Continuous simulation experiments under the aforementioned optimal conditions demonstrated that five-stage countercurrent extraction combined with four-stage countercurrent stripping effectively improved the overall process performance. The schematic diagram of the operation has been presented in Figure 11.

The phenomenon of phase separation during the continuous countercurrent extraction experiment.

As shown in Figure 11, during the continuous countercurrent extraction and stripping experiments, both the aqueous phase and the organic phase exhibited clear stratification and rapid separation, with no significant emulsification observed. In the five-stage continuous countercurrent extraction process, the color of the feed solution gradually lightened, transitioning from a deep blue aqueous phase to a pale blue raffinate, indicating a high vanadium recovery efficiency. The fresh organic phase progressively changed from being clear, transparent, and colorless to a deep blue, demonstrating that tetravalent vanadium ions were successfully transferred from the aqueous phase to the organic phase via cation exchange. Stripping, as the reverse reaction of extraction, displayed opposite trends in the four-stage continuous countercurrent stripping process, confirming an effective stripping performance and successful vanadium enrichment. The compositions of the raffinate and the stripping solution obtained from the continuous experiments involving five-stage countercurrent extraction and four-stage countercurrent stripping have been summarized in Table 5.

Table 5. The composition of the raffnate and stripping solution in continuous counter current extraction (mg/L).

Elements	V	Fe	Al	Zn	Na
Feed solution	56721.21	2776.54	10313.06	3871.25	711.19
Raffinate	2246.16	1755.05	7958.59	2764.8	33185.60
Stripping solution	108405.35	78.50	1144.28	79.42	65.25

As shown in Table 5, under the optimized parameter conditions, following the five-stage countercurrent extraction, the vanadium extraction efficiency reached 96.04%, while the extraction efficiencies of iron and aluminum impurities were 36.79% and 22.83%, respectively. Moreover, the concentration of vanadium in the stripping solution obtained through four-stage countercurrent stripping was as high as 108405 mg/L.

3.3. Performance characterization of the prepared vanadium electrolyte

3.3.1. Physical and chemical properties of prepared electrolytes

A standard electrolyte sample (SS) containing 2.0 mol/L V and 3.0 mol/L H₂SO₄ was prepared using VOSO₄∙xH₂O (99.5%) as a reference. Subsequently, a specific amount of H₂SO₄ was added to ES to ensure the SO₄^2- concentration matched that of SS. The electrical conductivity and viscosity of the electrolyte play a crucial role in determining the kinetics, reversibility, and resistance of electrode reactions, ultimately influencing charge transfer and reactant diffusion. The physicochemical parameters of ES and SS have been summarized in Table 6.

Table 6. The physical properties of ES and SS.

Samples	V (mol/L)	SO₄^2- (mol/L)	Electrical conductance (ms/cm)	Viscosity (mm²/s)
SS	2.0	5.0	269.1	5.01
ES	2.0	5.0	253.7	5.00

As shown in Table 6, the conductivities of ES and SS are relatively close, at 253.7 mS/cm and 269.1 mS/cm, respectively. The viscosities of SS and ES are nearly identical, being 5.01 mm²/s and 5.00 mm²/s, respectively. It is hypothesized that the slightly lower conductivity of ES may be attributed to differences in proton concentration. This discrepancy arises because H₂SO₄ serves dual roles as both the stripping agent and supporting electrolyte, with H+ ions being consumed during the stripping process due to their replacement by VO²⁺. These results demonstrate that the conductivity of vanadium electrolytes is highly dependent on their proton concentration, highlighting the necessity of adjusting the H₂SO₄ concentration in the electrolyte [29].

3.3.2. Electrochemical properties of prepared electrolytes

CV is a widely used technique for evaluating the electrochemical activity and reversibility of electrolytes. As evidenced by the CV data in Figure 12, the parameters listed in Table 7 include the oxidation peak current density (i_pa), reduction peak current density (i_pc), and peak potential difference (Ep). As shown in Table 7, the i_pa values of ES and SS are 98.366 mA/cm² and 99.512 mA/cm², respectively, while the ipc values of ES and SS are 73.702 mA/cm² and 76.631 mA/cm², respectively. We note that our values for the purified electrolyte (ES) fall well within the range reported for good-quality electrolytes, confirming that our preparation method successfully yields an electrochemically competent product. This contextualizes our findings within the broader field. These results suggest favorable reaction kinetics on the electrode surface of ES. The ΔEp values of ES and SS are 0.363 V and 0.361 V, respectively, and the i_pa/i_pc ratios of ES and SS are 1.335 and 1.299, respectively, indicating good reversibility of the electrode reactions in ES [30,31].

Table 7. Cyclic voltammetry test parameters of ES and SS.

Samples	Anodic peak		Cathodic peak		△E_p (V)	i_pa/i_pc
Samples	i_pa (mA/cm²)	E_pa (V)	i_pc (mA/cm²)	E_pc (V)	△E_p (V)	i_pa/i_pc
SS	99.512	1.139	76.631	0.778	0.361	1.299
ES	98.366	1.146	73.702	0.783	0.363	1.335
Ref [10]	108.30	1.259	80.04	0.752	0.507	1.353
Ref [20]	68.03	1.242	49.26	0.84	0.402	1.38
Ref [24]	114.40	1.098	79.95	0.685	0.413	1.43

The electrochemical reaction kinetics of the ES cathode electrolyte were further investigated using electrochemical impedance spectroscopy (EIS). As evidenced by the impedance spectroscopy (Figure 13) and the derived circuit simulation data (Table 8), the solution resistances (Rs) and charge transfer resistances (Rct) corresponding to the high-frequency region for ES and SS are 1.099 Ω·cm², 1.004 Ω·cm² and 0.2132 Ω·cm², and 0.1212 Ω·cm², respectively. The marginally reduced conductivity of ES exerts only minimal influence on charge transfer kinetics. In the low-frequency domain, the linear segment slopes directly reflect the diffusion coefficients (W) for both ES and SS systems, which are 0.2137 S/s⁵·cm² and 0.3609 S/s⁵·cm², respectively. These findings suggest that ES exhibits better diffusion characteristics and reduced concentration polarization compared to SS.

Table 8. Kinetic parameters of charge transfer process for ES and SS electrode reactions.

Sample	R_s (Ω/cm²)	C×10^-5 (F/cm²)	R_ct (Ω/cm²)	W,Y₀ (S/s⁵cm²)
SS	1.004	44.1	0.1212	0.3609
ES	1.099	58.5	0.2132	0.2137

The removal of Al³⁺ enhances electrochemical performance mainly by reducing electrolyte viscosity and minimizing unwanted side interactions at the electrode surface. Al³⁺ ions, particularly at high concentrations, increase electrolyte viscosity, which hinders the diffusion of vanadium ions to and from the electrode surface, leading to stronger concentration polarization as reflected by a larger peak potential separation in CV. After Al removal, the electrolyte viscosity decreases, improving vanadium ion diffusion and reducing peak potential separation, with a lower Warburg coefficient in EIS also indicating better ion diffusion properties. Additionally, though not directly involved in charge transfer, Al³⁺ can adsorb on the electrode surface, blocking active sites and slightly impeding electron transfer kinetics for vanadium redox reactions; its removal results in a cleaner electrode surface and a marginal reduction in charge transfer resistance, mitigating kinetic inhibition from impurities.

3.3.3. Charge-discharge tests

The redox reactions occurring during both charge and discharge cycles fundamentally govern vanadium battery performance. Consequently, a detailed analysis of single-cell operation becomes imperative [32]. Figure 14 compares the ES and SS systems through three key metrics: Coulombic efficiency (CE), voltage efficiency (VE), and overall energy efficiency (EE).

The efficiency of vanadium batteries with ES and SS as electrolytes.

As shown in Figure 14, the battery efficiency remains largely stable with increasing cycle numbers. In the 80-cycle charge and discharge test, the average Coulombic efficiencies of SS and ES were 86.08% and 87.86%, respectively; the average voltage efficiencies of SS and ES were 94.44% and 93.00%, respectively, confirming the higher ohmic polarization of ES due to its lower voltage efficiency; the average energy efficiencies of SS and ES were 81.29% and 81.70%, respectively. The about 93% VE (and consequently about 81% EE) achieved is actually very good for a lab-scale cell and a complex feedstock-derived electrolyte. Achieving EE >85% typically requires further optimization of operating conditions (e.g., lower current density, optimized flow fields), membrane properties, and electrode activation.

Our electrolyte’s performance (Avg. CE: 87.86%, VE: 93.00%, EE: 81.70% over 80 cycles @ 40 mA/cm²) is comparable to, and in some cases superior to, many electrolytes prepared from purified V₂O₅ or other secondary sources reported in studies using similar testing conditions. This comparison underscores the high quality and practical potential of our short-process electrolyte derived from a challenging high-aluminum feedstock [24,33-36]. Thus, the single-cell test results reveal nearly identical performance characteristics for ES and SS systems.

3.3.4. Raman characterization of the electrolyte solution

Raman spectroscopy has been employed to investigate the chemical composition of V(IV) sulfate solutions and the characteristics of vanadium-vanadium and vanadium-sulfate complexes formed in these solutions [37]. To elucidate the coordination interactions between VO²⁺ and SO₄^2-, as well as HSO₄^-, Raman spectroscopic analyses were conducted on ES and SS, with the results shown in Figure 15. As depicted in Figure 15, the Raman spectra of ES and SS are nearly identical, indicating that the complexation forms of vanadium and sulfate species are highly similar. This observation aligns well with the electrolyte test results.

The Raman spectra of ES and SS within the range of 250 - 1250 cm-1.

4. Conclusions

This study systematically investigated the effects of aluminum impurity ions on the preparation process and performance of vanadium electrolyte, establishing the upper limit of aluminum concentration in the electrolyte. Aluminum impurities were pre-removed in the form of ammonium alum crystals by adding ammonium sulfate to the vanadium-rich solution prior to extraction. Under optimal conditions (m(NH₄⁺):m(Al³⁺) = 1:1, temperature 5°C, and standing time 6 h), an aluminum removal rate of 51.67% was achieved with negligible vanadium loss. Subsequently, sodium sulfite was introduced to reduce iron ions in the feed solution, effectively suppressing their oxidation and co-extraction during the extraction process. These pretreatment measures significantly minimized the co-extraction of impurities, thereby enhancing the utilization efficiency of the organic phase and improving product purity.

After parameter optimization, five-stage countercurrent extraction (60% saponification, pH 1.0, 30% D2EHPA, 3:1 O/A, 8 min) achieved 96.04% vanadium recovery. Four-stage stripping (6:1 O/A, 4 M H₂SO₄, 20 min) completed the separation process. The optimized separation protocol successfully removed aluminum and iron contaminants while simultaneously increasing vanadium concentration by a significant margin.

The 2.0 mol/L VOSO₄ solution (ES) prepared via the crystallization and re-extraction process exhibited physical and chemical properties, electrochemical performance, and battery charge-discharge characteristics comparable to those of the standard sample (SS). The system exhibited superior redox activity and cycling stability, maintaining average efficiencies of 87.86% (CE), 93.00% (VE), and 81.70% (EE) through 80 consecutive charge-discharge cycles at 40 mA/cm². These performance metrics confirm its viability as an advanced vanadium-based electrolyte for flow battery applications.

Acknowledgment

This work was financially supported by Natural Science Foundation of Hubei Province of China (2023AFB061).

CRediT authorship contribution statement

Hong Liu: Conceptualization, Investigation, Methodology, Writing-original draft, Writing-review & editing. Tao Liu: Writing-review & editing. Yimin Zhang: Visualization, Supervision. Hongru Qu: Investigation, Methodology. Youwen Hu: Investigation, Writing – review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Declaration of generative AI and AI-assisted technologies in the writing process

The authors confirm that there was no use of artificial intelligence (AI)-assisted technology for assisting in the writing or editing of the manuscript and no images were manipulated using AI.

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1. Introduction

2. Materials and Methods

2.1. Reagents and raw materials
2.2. Methods

3. Results and Discussion

3.1. Regulation of Al impurities in vanadium-rich solution
3.2. Preparation of vanadium electrolyte by saponified D2EHPA
3.3. Performance characterization of the prepared vanadium electrolyte

4. Conclusions

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[1] Karami T., Zangeneh M.R.. Performance enhancement of vanadium redox flow battery by flow field modification: Channel height reduction and channel blockage. Future Batteries. 2025;7:100086. https://doi.org/10.1016/j.fub.2025.100086
[Google Scholar]

[2] Beriwal N., Sharma L., Verma A.. Powering a vanadium redox flow battery using spent vanadium catalyst: Extraction of direct-use V(IV)/V(III) vanadium precursors. Journal of Cleaner Production. 2023;429:139568. https://doi.org/10.1016/j.jclepro.2023.139568
[Google Scholar]

[3] Yang L., Fan Z., Cui F., Wu T., Fang T., Guo Y., Tian L., Pang B., He G., Wu X.. Atomic Bi active sites decorated graphite felt derived from bismuth-based metal−organic framework boosting vanadium redox flow battery performance. Electrochimica Acta. 2025;526:146190. https://doi.org/10.1016/j.electacta.2025.146190
[Google Scholar]

[4] Zorrilla F., Montiel M., Mustata R., Losantos R., Valiño L.. Impact of electrolyte composition on the mitigation of electrolyte imbalance in a vanadium redox flow battery: A 3D multiphysics model. Journal of Energy Storage. 2025;107:114899. https://doi.org/10.1016/j.est.2024.114899
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[5] Ali A., Ramadesigan V., Monder D.S.. Modelling and simulation based impact analysis of electrode parameters on the performance of vanadium redox flow batteries. Journal of Power Sources. 2025;653:237659. https://doi.org/10.1016/j.jpowsour.2025.237659
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[6] Tang H., Shi K., Wang Y., Yang H., Wan W., Su Y.H., Li C.. Design and technical assessment of photovoltaic and vanadium redox flow battery systems for residential buildings based on time-of-use electricity pricing strategy. Energy Conversion and Management. 2025;341:120059. https://doi.org/10.1016/j.enconman.2025.120059
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[7] Yu Z., Jia X., Cai Y., Su R., Zhu Q., Zhao T., Jiang H.. Electrolyte engineering for efficient and stable vanadium redox flow batteries. Energy Storage Materials. 2024;69:103404. https://doi.org/10.1016/j.ensm.2024.103404
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[8] Long H., Liao C., Xie C., Li X.. Highly stable electrolyte enables wide temperature vanadium flow batteries. Journal of Energy Chemistry. 2025;106:1038-1045. https://doi.org/10.1016/j.jechem.2025.05.007
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[9] Cecchetti M., Nicosia G., Casalegno A., Zago M.. Mitigation of capacity decay in vanadium redox flow batteries through initial imbalance of electrolytes state of charge. Electrochimica Acta. 2025;517:145750. https://doi.org/10.1016/j.electacta.2025.145750
[Google Scholar]

[10] Hu Y.W., Zhang Y.M., Liu T., Liu H.. Clean and cost-efficient preparation of vanadium electrolyte from the vanadium-rich solution of black shale by solvent extraction. Journal of Cleaner Production. 2023;394:136389. https://doi.org/10.1016/j.jclepro.2023.136389
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[11] Zheng N., Qiu X., Jin S., Xu Z., Zhou M., Zhou Z., Zhou T., Sun Z.. Preparation of V4+electrolyte by nanofluid-based electrocatalytic reduction of V2O5 for vanadium redox flow batteries. Electrochimica Acta. 2025;513:145532. https://doi.org/10.1016/j.electacta.2024.145532
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[12] Zhang Y., Dreisinger D., Zhang T.A., Lv G., Zhang W., Xie F.. Preparation of highly pure vanadyl sulfate electrolyte from vanadium slag leach solution with the complexing effect of EDTA on Fe(III) Hydrometallurgy. 2019;188:54-63. https://doi.org/10.1016/j.hydromet.2019.05.012
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[13] Hsiao Y.S., Huang J.H., Lin H.Y., Huang J.H., Weng L.Y., Cheng T.H., Pang W.K., Hsu S.C., Weng H.C., Huang Y.C.. Recovery of V₂O₅ from spent catalysts and its application in vanadium electrolytes for vanadium redox flow batteries. Journal of Energy Storage. 2025;116:115990. https://doi.org/10.1016/j.est.2025.115990
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