Low-carbon upcycling of vanadium slag into doped cathodes for high-performance zinc batteries

Abstract

Developing sustainable aqueous energy storage systems is crucial for advancing renewable energy utilization. Herein, a short-process strategy that integrates vanadium metallurgy and material preparation to synthesize high-performance cathodes is proposed for aqueous zinc batteries. By selectively removing harmful impurities from vanadium-slag leachate while utilizing beneficial impurities as dopants, NH₄⁺-intercalated and metal-doped V₂O₅ (NHVO-M_x) is efficiently synthesized. The resulting material exhibits enhanced Zn²⁺ diffusion kinetics due to its expanded interlayer spacing and low crystallinity structure, while its reduced bandgap significantly accelerates electron transfer. It delivers a high specific capacity of 454.4 mAh g⁻¹ at 0.1 A g⁻¹ and maintains 86.6% capacity retention after 3000 cycles at 8 A g⁻¹. Furthermore, this material is employed in a pouch cell, achieving a capacity exceeding 0.39 Ah. This innovative approach reduces costs by 40% and lowers carbon emissions by over 65% by efficiently utilizing inherent impurities instead of relying on conventional chemical additives. It not only simplifies the purification process but also enhances the battery's capacity and sustainability. This work establishes a green, streamlined synthesis paradigm for next-generation aqueous batteries by converting industrial waste impurities into valuable functional components.

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Broader context

With the rise of renewable energy, there is an increasing need for efficient and environmentally friendly battery technologies. Aqueous Zn–V-based batteries, known for their safety and low cost, present a promising solution. However, traditional vanadium-based cathodes face inherent limitations, including sluggish zinc-ion diffusion and low electronic conductivity, which result in poor energy storage performance and cycling stability. Although optimization strategies such as doping and interface engineering have yielded some improvements, zinc-ion batteries still fall short for practical applications and often involve complex, costly, and environmentally harmful synthesis. In this work, we propose an innovative, low-carbon synthesis method that abandons the traditional “vanadium purification-doping” process; instead it efficiently utilizes inherent impurities from vanadium sources as dopants. This method results in a novel vanadium-based cathode material, featuring ammonium-ion intercalation and multi-metal doping (denoted as NHVO-M_x). Thanks to the NH₄⁺-regulated interlayer spacing and multi-metal-introduced band structure engineering, the material exhibits enhanced structural stability and improved reaction kinetics, leading to high capacity and stable performance. Furthermore, our approach significantly reduces both manufacturing costs and carbon emissions, representing a major step forward in both high-performance Zn-ion battery cathode design and low-carbon material synthesis.

Introduction

The accelerated advancement and efficient utilization of renewable energy have been recognized as critical pathways to realize the strategic goal of carbon neutrality.^1,2 Concurrently, the demand for safe and environmentally friendly energy storage systems is rising, underscoring the need for the development of high-energy-density batteries derived from sustainable resources.³ Aqueous zinc-ion batteries (AZIBs) are gaining attention as promising candidates for the next generation of energy storage systems, due to their enhanced safety, low cost, competitive performance, and environmental friendliness.^4,5 The exploration of cathode materials with excellent structure, favourable stability, and high capacity is critical to the progress of AZIBs.^5–7 Various materials have been considered as cathodes for AZIBs, including manganese oxides, Prussian blue analogues, vanadium-based materials, and organic compounds.^8–14 Among these, vanadium oxides are particularly noted due to their rich valence states (V⁵⁺ ↔ V²⁺) and open framework structure, which contribute to their high theoretical capacity and considerable potential for application.^15,16 However, the performance of vanadium pentoxide is hindered by its intrinsic low conductivity and sluggish diffusion kinetics.^17,18 Recent studies have shown that ion pre-intercalation or doping of the host is an effective strategy to modify its electronic and structural properties, ultimately improving the capacity and stability of AZIBs.^19,20

The doping of various metal cations, such as Li⁺, Na⁺, Mg²⁺, Ca²⁺, Co²⁺, Al³⁺, and Mn²⁺, into vanadium-based cathodes has been demonstrated to significantly enhance their performance.^21–28 Compared to single-cation modification, multi-ion doping has been shown to produce synergistic effects.^29,30 Moreover, the design of new materials places less emphasis on economic feasibility and environmental effects. The conventional synthesis methods for doping-modified vanadium-based materials typically use commercial high-purity vanadium oxide as the vanadium source, with metal salts acting as doping agents.^21,31 Therefore, it is encompassed within the life cycle of the cathodes, from vanadium slag leachate to high-purity vanadium, involving a sequence of vanadium precipitation, filtration, and re-dissolution via heating, or alternatively, a multi-stage solvent extraction process, followed by washing and stripping.^32–34 From a comprehensive perspective, the traditional synthesis strategy involves a lengthy process, high carbon emissions, and considerable environmental concerns. Additionally, the deep impurity removal and doping stages require the use of numerous chemicals, leading to increased costs. Therefore, developing cost-effective and environmentally sustainable preparation methods with shorter processes for vanadium-based cathodes is necessary.

Given the similarity between doping ions and impurities in leachate, we propose selectively utilizing beneficial impurities as dopants in the synthesis of vanadium oxides, thereby enabling a short, low-carbon route for producing doped vanadium-based cathodes. This process involves targeted impurity removal from vanadium slag leachate, without stringent standards for beneficial impurities, followed by precipitation and low-temperature calcination. The strategy successfully synthesizes V₂O₅ doped with multiple elements such as Cr, Ca, and K, and intercalated with NH₄⁺ (denoted as NHVO-M_x). These modifications reduce the band gap, significantly accelerating electron transfer, which is crucial to enhance electrochemical performance. Additionally, the NH₄⁺-induced expansion of interlayer spacing and the low crystallinity structure further enhance diffusion kinetics at the cathode, contributing to superior (dis-)charge efficiency. Notably, the NHVO-M_x cathode demonstrates a high specific capacity of 454.4 mAh g⁻¹ at 0.1 A g⁻¹ and excellent cycling stability, retaining 86.6% of its capacity after 3000 cycles at 8 A g⁻¹. Moreover, this material is also used in a pouch cell, delivering a capacity over 0.39 Ah. A multi-objective assessment from both economic and environmental perspectives reveals that the short-process route not only reduces costs and environmental impact but also lowers carbon emissions compared to conventional cathode design.

Results and discussion

Low-carbon short-process synthesis of doped vanadium oxides

The analysis of the alkaline leachate from vanadium slag by inductively coupled plasma optical emission spectrometry (ICP-OES) revealed the coexistence of multiple impurity elements such as V, Cr, Si, and K (Table S1, SI). In conventional synthesis of doping-modified vanadium oxides, vanadium slag leachate undergoes a multi-step deep impurity removal process to produce high-purity vanadium, which is then doped with external dopants and subjected to hydrothermal synthesis (Fig. S1a, SI). However, this process requires a long process and multiple reagents, resulting in high emissions. We propose a novel approach that utilizes the impurities as natural dopants, which are potentially beneficial for energy storage performance, integrating metallurgy and material synthesis. This method synthesizes vanadium oxides with efficient metal ion doping through a low-carbon emission, short-process route (Fig. 1a and Fig. S1b, SI). To assess whether these elements are advantageous within their natural concentration ranges in the leachate, energy storage performance tests were conducted on materials synthesized via vanadium precipitation and low-temperature calcination from simulated leachates containing each element individually (as detailed in Experimental section, SI). Compared to the (NH₄)_0.24V₂O₅ (denoted as NHVO) synthesized from deeply purified leachate, the energy storage performance of NHVO-Cr, NHVO-Ca, and NHVO-K (synthesized from leachates containing Cr, Ca, and K, respectively, in their actual concentration ranges) was significantly superior (Fig. S2, SI). For example, these materials showed a higher first-cycle capacity compared to NHVO at 0.2 A g⁻¹, and their capacity improved after rate capability testing when the current density was returned to 0.2 A g⁻¹ (Fig. 1b and c). However, the performance of NHVO-Si, synthesized from Si-containing leachate, significantly declined as the silicon concentration increased. When the Si concentration exceeded 0.1 g L⁻¹, the capacity at 0.2 A g⁻¹ notably decreased after rate capability testing (Fig. 1c), suggesting that excessive silicon negatively impacts performance. To address this, the Si concentration in the leachate can be reduced to below 0.02 g L⁻¹ using a desilication agent, while other main metal ions remain largely unchanged (Table S2, SI). The resulting multi-element co-doped NHVO (NHVO-M_x), derived from a desiliconized solution with average multi-element concentrations, exhibits superior energy storage capacity compared to the individual doping of each element, achieving 439.1 mAh g⁻¹ at 0.2 A g⁻¹ (Fig. 1d).

Fig. 1 Short-process synthesis of NHVO-M_x. (a) Schematic of the low-carbon short-process synthesis from vanadium slag leachate to NHVO-M_x. (b) First-cycle capacities of NHVO-Cr, NHVO-Si, NHVO-Ca, and NHVO-K, synthesized from leachates containing Cr, Si, Ca, and K within actual concentration ranges, respectively, as cathodes for AZIBs at 0.2 A g⁻¹. (c) The change in capacity at 0.2 A g⁻¹ before and after the rate test for NHVO-Cr, NHVO-Si, NHVO-Ca, and NHVO-K. (d) Rate performance of NHVO-M_x synthesized from leachate after silica removal. (e) XRD patterns of vanadium precipitation precursors from leachates with and without impurity metal ions. (f) XRD patterns of NHVO(-M_x) obtained by calcination of vanadium precipitation precursors at 300 °C under a nitrogen atmosphere, and V₂O₅(-M_x) obtained by calcination at 600 °C under an air atmosphere. (g) Schematic of NHVO and V₂O₅  obtained from precursors NH₄VO₃ under different calcination conditions.

Furthermore, the multi-element co-doped cathodes prepared at varying actual concentrations demonstrated equally impressive performance, showcasing the flexible concentration requirements of the leachate's beneficial impurities (Table S3 and Fig. S3, SI). Therefore, the first step focuses on targeted desilication of the leachate, without the need for deep impurity removal or achieving the high-purity standards listed in Table S4 (SI), significantly reducing reagent and energy consumption during the purification process. Subsequently, an ammonium salt is added to the desilicated leachate to precipitate vanadium, yielding NH₄VO₃ doped with various elements (NH₄VO₃-M_x). X-ray diffraction (XRD) shows that all diffraction peaks are consistent with NH₄VO₃, indicating that the trace multi-element doping does not significantly alter the NH₄VO₃ crystal structure (Fig. 1e). In the last calcination step, to determine the optimal conditions, we investigated the structure and performance of vanadium oxides obtained from NH₄VO₃ under different conditions. The XRD results reveal that the product calcined at 600 °C under an oxygen atmosphere, under conventional vanadium product calcination conditions, is orthorhombic V₂O₅ with an interplanar spacing of 4.4 Å for the (001) plane. In contrast, the product calcined at 300 °C under a nitrogen atmosphere is NHVO with some NH₄⁺ retained, exhibiting an expanded plane spacing of 12.3 Å, along with improved performance compared to V₂O₅ (Fig. 1f and g and Fig. S4, SI). Moreover, Tables S5, 6 and Fig. S5–7 (SI) provide a detailed analysis of how calcination temperature (250 °C, 300 °C, 350 °C) and duration (1.5 h, 2 h, 3 h) affect the crystal structure and energy storage performance of vanadium oxides. The results indicate that calcining in nitrogen at 300 °C for 2 hours yields vanadium oxides with optimal NH₄⁺ retention and enhanced stability. Additionally, this low-temperature calcination, as opposed to the high-temperature calcination typically used in V₂O₅ synthesis, reduces energy consumption and carbon emissions. Thus, we propose an environmentally friendly synthesis route for doped-vanadium oxide materials derived from vanadium slag leachate. This method involves three steps: (1) selective removal of Si from the vanadium slag leachate, (2) addition of ammonium salts to produce multi-element doped NH₄VO₃, and (3) low-temperature calcination under nitrogen to form vanadium oxides intercalated with NH₄⁺ and doped with multiple elements (NHVO-M_x). Compared to traditional methods for synthesizing doped vanadium oxides, this approach significantly shortens the process, reduces the need for large quantities of reagents and energy, and lowers both carbon emissions and costs.

Crystal structure and electronic structure

The XRD patterns in Fig. 1f reveal that the diffraction peaks of NHVO-M_x are consistent with those of NHVO. A prominent diffraction peak appears at 7.1°, and another peak is observed at its integer multiple, 14.2°, indicating that NHVO-M_x possesses a well-defined layered structure with a substantial interlayer spacing of 12.3 Å. This expansion of the interlayer spacing is attributed to the intercalation of NH₄⁺ caused during low-temperature calcination.³⁵ The increased interlayer distance facilitates the reversible intercalation and extraction of zinc ions. To further investigate the incorporation of NH₄⁺ between the layers, Raman spectroscopy and Fourier-transform infrared spectroscopy (FT-IR) were performed. Raman spectra showed a slight redshift in the V=O stretching vibration and V–O–V interlayer bending vibration peaks for NHVO-M_x compared to V₂O₅, indicating a subtle elongation of the V=O bonds along the c-axis (Fig. 2a). Additionally, FT-IR analysis identified a characteristic NH₄⁺ peak, and the redshift in the V=O and V–O–V vibration peaks, suggesting the insertion of NH₄⁺ between the layers and interactions with lattice oxygen (Fig. 2b). Scanning electron microscopy (SEM) images and energy dispersive X-ray spectroscopy (EDS) showed the micron-sized NHVO-M_x with a uniform distribution of V, O, N, and Cr, supporting the successful intercalation of NH₄⁺ and doping with metal ions (Fig. 2c). However, some trace metal elements were present in such low amounts that they were undetectable by EDS. To obtain quantitative data on the doped elements in the NHVO-M_x, the material was dissolved and analysed by ICP. The atomic ratios of Cr/V, Ca/V, and K/V were 0.72%, 0.19% and 0.37%, respectively, confirming that the primary impurity elements from the leachate were successfully doped into the NHVO-M_x for use as cathodes in aqueous zinc batteries (Fig. 2d).

Fig. 2 Structural characteristics of NHVO-M_x. (a) Raman spectra of NHVO(-M_x) and V₂O₅(-M_x). (b) FT-IR spectra of NHVO-M_x and V₂O₅. (c) SEM image and EDS mapping of NHVO-M_x. (d) The atomic ratios of impurity metals to vanadium are calculated from the elemental concentrations measured by ICP-OES after the complete dissolution of NHVO-M_x.

As observed in X-ray photoelectron spectroscopy (XPS), some of the V⁵⁺ ions were reduced to V⁴⁺ in NHVO-M_x to maintain electrical neutrality. The peaks at 516.1 and 517.4 eV correspond to the V 2p_3/2 electrons of V⁴⁺ and V⁵⁺ respectively (Fig. 3a). This partial reduction of V⁵⁺ promoted electron transfer. The high-resolution XPS showed that the characteristic peak of lattice oxygen in the O 1s spectrum shifted from 529.8 eV to 530.2 eV, suggesting an interaction between the interlayer NH₄⁺ and the lattice oxygen (Fig. 3b). Additionally, the doping of guest elements typically leads to the formation of oxygen vacancies, as indicated by the peak at 531.4 eV in the O 1s spectrum. Based on the peak area, the proportion of oxygen vacancies in NHVO-M_x was found to increase to 31.4%. To further confirm the oxygen vacancy structure, Electron Paramagnetic Resonance (EPR) measurements were performed. As shown in Fig. 3c, V₂O₅ exhibits a weak signal, while NHVO and NHVO-M_x display stronger symmetric peaks at g ≈ 2.00, corresponding to the characteristic signal of oxygen vacancies. This indicates that the intercalation of ammonium ions and the doping of metals promote the increase of oxygen vacancies. EPR tests were also conducted on NHVO with individual metal doping, and it was found that the oxygen vacancy concentration increased more significantly with the doping of K (Fig. S8, SI). The sample doped with Cr showed a broader characteristic peak, which may be attributed to the changes in the local electronic environment of the oxygen vacancies. These oxygen defects reduced the interaction between zinc ions and the lattice, thereby facilitating ion diffusion. In high-resolution transmission electron microscopy (HRTEM), V₂O₅ exhibited clear lattice fringes and narrow spacings (Fig. 3d). NHVO-M_x with intercalated NH₄⁺ showed an interlayer spacing of 12.3 Å and a polycrystalline structure of low crystallinity (Fig. 3e). Moreover, the selected area electron diffraction (SAED) pattern of NHVO-M_x displayed concentric rings formed by multiple points, corresponding to a polycrystalline structure. In contrast, the diffraction pattern of V₂O₅ consisted of bright diffraction spots, indicating a high degree of crystallinity (Fig. S9, SI). This suggested that NHVO-M_x possessed a more disordered crystal structure. The low crystallinity of NHVO-M_x provided additional diffusion pathways and active sites, effectively alleviating the structural stress caused by ion intercalation.³⁶

Fig. 3 Structure and electronic structure of NHVO-M_x. (a and b) High-resolution XPS spectra of NHVO-M_x: (a) V 2p and (b) O 1s. (c) EPR spectra of NHVO(-M_x). (d and e) HRTEM images of (d) V₂O₅ and (e) NHVO-M_x. (f) Valence band structures of NHVO(-M_x) and V₂O₅(-M_x). (g) Energy band structures of NHVO(-M_x) and V₂O₅(-M_x). (h and i) Differential charge densities with Zn-ion intercalation in (h) V₂O₅ and (i) NHVO-M_x. The yellow and cyan regions represent charge accumulation and depletion, respectively. The isosurface value is 0.003 e per bohr³.

To further investigate the effects of NH₄⁺ and various metal ions on the electronic structure of the material, XPS valence band measurements were performed. We analysed the relative positions of the valence band (VB) maxima for V₂O₅, V₂O₅-M_x, NHVO, and NHVO-M_x. Metal doping shifted the valence band edge of V₂O₅ from 2.50 eV to 2.39 eV and the valence band edge of NHVO from 2.80 eV to 2.65 eV, thereby enhancing the maximum energy edge (Fig. 3f). To further elucidate the electronic properties at the interface, we employed UV–visible diffuse reflectance spectroscopy (UV-vis DRS) to investigate the band structure of V₂O₅ with and without NH₄⁺ intercalation and metal doping. V₂O₅ without doping or intercalation exhibited the widest band gap. NH₄⁺ intercalation led to a reduction in the overall band structure and narrowed the band gap. Compared to the 1.78 eV bandgap of NHVO, doping with different metals further reduced the bandgap, with Ca metal doping contributing a more significant reduction. Additionally, co-doping with multiple metal ions generates a synergistic effect, resulting in a narrower bandgap of 1.4 eV for NHVO-M_x (Fig. 3g and Fig. S10, 11, SI). The role of NH₄⁺ intercalation and metal doping in modulating the electronic structure was further investigated using Density Functional Theory (DFT). The total densities of states of V₂O₅ and NHVO-M_x are shown in Fig. S12 (SI). Compared to V₂O₅, NHVO-M_x exhibited a narrower band gap, and the Fermi level shifted from the top of the valence band to higher density states. This suggested that the intercalation of NH₄⁺ and metal doping improve the electronic conductivity, which is beneficial for electron transfer during electrochemical reactions. The differential charge density showed the interaction between the cathode and the inserted zinc ions. Fig. 3h and i show that V₂O₅ demonstrated a strong interaction with the Zn²⁺, indicating that the insertion/extraction of Zn²⁺ was relatively difficult, with slow diffusion rates. In contrast, the improvement in crystal and electronic structures significantly weakened the electrostatic interaction between NHVO-M_x and Zn²⁺, which benefited the zinc ion diffusion dynamics. In conclusion, the proposed eco-friendly short-process strategy effectively utilizes metal ion doping and NH₄⁺ intercalation to improve both the crystal and electronic structures, promoting charge transfer and ion diffusion, thus benefiting NHVO-M_x with excellent electrochemical performance.

Kinetic mechanism and zinc storage performance

Intercalation and element doping can modify the structure and electronic properties of metal oxides. The resulting increase of interlayer spacing and low crystallinity structures facilitate ion diffusion, and the reduced bandgap accelerates charge transfer, which in turn boosts electrochemical activity. To verify the enhancement of ion transport properties in the electrode, the Zn²⁺ diffusion coefficients (D_Zn²⁺) of cathodes were measured using the galvanostatic intermittent titration technique (GITT, Fig. S13a, SI). The results indicated that the D_Zn²⁺ in the NHVO-M_x cathode ranged from 10⁻⁹ to 10⁻¹¹ cm² s⁻¹, which was slightly higher than that in NHVO and significantly higher than that in the V₂O₅ cathode (10⁻¹⁰ to 10⁻¹³ cm² s⁻¹) (Fig. 4a). This suggested that the enhanced ion diffusion was primarily attributed to the increased interlayer spacing caused by the intercalation of NH₄⁺. The doping of metal ions also played a role in enhancing the diffusion of zinc ions within the material to a certain extent (Fig. S13b, SI). To further investigate the effects of metal ion doping on the ionic diffusion and electron transfer properties of NHVO-M_x electrodes, electrochemical impedance spectroscopy (EIS) was conducted. The semicircle in the high-frequency region corresponds to charge transfer resistance, while the linear tail in the low-frequency region represents the ionic diffusion process. An equivalent circuit was fitted to the test results, which includes solution resistance (R_s), double-layer capacitance (Q), charge transfer resistance (R_ct), and diffusion impedance (W_o) (Fig. 4b). It was found that NHVO-M_x had an R_ct of 62.7 Ω, whereas the undoped NHVO electrode exhibited a higher R_ct of 294 Ω. This indicates that doping with multiple metal ions of Cr, Ca, and K reduces the charge transfer resistance of the cathode. The steep slope in the low-frequency region and the low diffusion impedance further confirm the superior ionic diffusion capability of NHVO-M_x. The results from the GITT and EIS tests indicate that the incorporation of NH₄⁺ into NHVO-M_x primarily enhances D_Zn²⁺, while multi-metal doping primarily lowers its R_ct. The combination of these factors improves charge transfer at the interface and ionic diffusion within the material, leading to an outstanding electrochemical performance.

Fig. 4 Electrochemical performance and kinetics of Zn/NHVO-M_x batteries. (a) Zinc ion diffusion coefficients of NHVO-M_x, NHVO, and V₂O₅ calculated from GITT results. (b) EIS curves of NHVO-M_x and NHVO. (c) Typical CV curves of NHVO-M_x and NHVO at a scan rate of 0.4 mV s⁻¹. (d) Peak current of NHVO-M_x versus scan rate. (e) Rate performance of NHVO-M_x and NHVO. (f) Comparison of the performance of NHVO-M_x with other vanadium-based cathodes from the literature. (g) Stability test of NHVO-M_x and NHVO at 8 A g⁻¹. (h) The performance of NHVO-M_x in soft-pack batteries.

Further analysis of the cyclic voltammetry (CV) curves of NHVO-M_x was conducted to examine the electrochemical kinetics of Zn²⁺ storage. In Fig. 4c, at a scan rate of 0.2 mV s⁻¹, the two reversible redox pairs at 0.56/0.75 V and 0.84/1.07 V correspond to the two-step (de)intercalation of Zn²⁺ during charge storage. The reduction peaks at 0.84 V and 0.56 V reflect the conversion of V⁵⁺ to V⁴⁺ and V⁴⁺ to V³⁺, respectively, with simultaneous Zn²⁺ insertion. The oxidation peaks correspond to the reverse processes, where vanadium is oxidized from V³⁺ to V⁴⁺ and then to V⁵⁺, and Zn²⁺ is de-inserted from the cathode. As shown in Fig. S14a (SI), when the scan rate increased from 0.2 mV s⁻¹ to 1.2 mV s⁻¹, the CV curves retained their original shape. Due to polarization, the oxidation peaks shifted to higher voltages and the reduction peaks shifted to lower voltages. The relationship between the peak current (i) and the scan rate (v) generally follows eqn (1):

i = av^b (1)

Here, a and b are adjustable. In NHVO-M_x, the b values for the oxidation peak A and the reduction peak B are 0.79 and 0.69, respectively (Fig. 4d). Therefore, the Zn²⁺ storage process in NHVO-M_x is primarily controlled by a mixed mechanism of capacitive response and diffusion, indicating high reaction kinetics. Additionally, the relative contributions of capacitance-controlled (k₁) and diffusion-controlled (k₂) behaviour can be quantitatively determined using eqn (2):

i(V) = k₁v + k₂v^1/2 (2)

As shown in Fig. S14b (SI), the pseudocapacitive contribution to the current increases with the scan rate, rising from 48.02% at 0.4 mV s⁻¹ to 67.72% at 1.2 mV s⁻¹, demonstrating the excellent high-rate performance of NHVO-M_x. The prominent surface capacitive contribution is likely due to the low crystallinity structure and larger interlayer spacing, which enhance the utilization of the electrochemically active surface. Moreover, the intercalation of NH₄⁺ and doping of metal ions into NHVO-M_x reduces its bandgap, improving interface electron transfer and resulting in remarkable high-rate performance and rapid charge–discharge capabilities.

The performance of Zn²⁺ storage in NHVO-M_x was evaluated using a coin-type half-cell. The galvanostatic charge/discharge (GCD) curves of NHVO-M_x at 0.1 A g⁻¹ displayed two sloping plateaus, corresponding to the oxidation and reduction peaks observed in the CV curves (Fig. S15, SI). The specific capacity of the NHVO-M_x cathode increased from 398.1 mAh g⁻¹ in the first cycle to 454.4 mAh g⁻¹ in the tenth cycle, indicating excellent energy storage capacity. The significant increase in capacity is primarily attributed to the activation of active materials during the cycling process, along with the gradual infiltration of electrolyte into the interlayers, which enhances the utilization of internal active materials.^37,38 Fig. 4e and Fig. S4 (SI) show the rate performance of the NHVO-M_x, NHVO, and V₂O₅ cathodes. It is evident that NH₄⁺ insertion and multi-element doping significantly enhance both the capacity and rate performance of the electrode. At current densities of 0.2, 0.5, 1.0, 2.0, 4.0, 6.0, and 8.0 A g⁻¹, the reversible discharge capacities of the NHVO-M_x were 439.1, 404.8, 371, 327.5, 281.4, 254.5, and 235.7 mAh g⁻¹, respectively. After the rate test, when the current density was returned to 0.2 A g⁻¹, the discharge capacity increased to 464.1 mAh g⁻¹, showing excellent capacity retention. Notably, the synergistic effects of multi-metal doping and NH₄⁺ intercalation enhanced the electrochemical kinetics of vanadium oxides, leading to improved and stabilized Zn²⁺ storage and demonstrating superior performance compared to other reported doped vanadium-based cathodes (Fig. 4f).^{22,23,29,30,39–45} The Ragone plot showed that the NHVO-M_x delivered stable and superior energy density across a wide range of power densities in aqueous zinc-ion batteries (Fig. S16, SI). For instance, it achieved an energy density of 298.0 Wh kg⁻¹ at a power density of 778.1 W kg⁻¹. Cycling stability tests further demonstrated that the NHVO-M_x electrode retained 86.6% of its capacity after 3000 cycles at 8.0 A g⁻¹ (Fig. 4g). In contrast, the V₂O₅ electrode underwent rapid degradation during cycling (Fig. S17, SI). This suggested that the large interlayer spacing with NH₄⁺ intercalation and low crystallinity structure, produced by low-temperature calcination, alleviated structural stress from zinc ion (de)intercalation, thus maintaining a stable framework. The long-term cycling performance at 0.2, 0.5, and 1 A g⁻¹ showed that NHVO-M_x retained 94.4%, 96.0%, and 93.8% of its capacity after 100, 250, and 600 cycles, respectively (Fig. S18, SI). Fig. 4h shows the performance of the NHVO-M_x material assembled in a pouch cell, demonstrating a capacity of 389.4 mAh at 0.2 A g⁻¹. When the current density increases to 0.5 A g⁻¹, the material retains a capacity of 321.6 mAh.

To investigate the electrochemical stability of NHVO-M_x and its zinc storage mechanism during cycling, ex situ X-ray diffraction (XRD) was employed to observe the structural evolution of the cathode during charge and discharge. The main structure of NHVO-M_x remained unchanged during cycling, indicating that the primary structure for Zn²⁺ storage was stable. Characteristic peaks shifted to lower 2θ values during discharge and returned to higher 2θ values during charge, indicating reversible insertion and extraction of Zn²⁺ (Fig. 5a). A weak Zn₃(OH)₂V₂O₇·2H₂O (ZVO) peak appeared at 0.2 V during discharge and disappeared at 1.6 V during charge, which may be related to reversible (de-)insertion of protons.⁴⁶ Further analysis revealed that single-layer orthorhombic V₂O₅, with low initial capacity, underwent electrochemical activation to transform into Zn_xV₂O₅·nH₂O, expanding the interlayer spacing from 4.4 Å to 11.8 Å and increasing capacity (Fig. S19 and 20, SI). However, the accumulation of zinc ions between the layers induced significant structural stress, leading to a rapid decline in performance during cycling. In contrast, NHVO-M_x, with NH₄⁺ ions, maintained a large interlayer spacing and a stable, open framework structure, resulting in excellent cycling stability.

Fig. 5 Energy storage mechanism of NHVO-M_x. (a) Ex situ XRD patterns during the charging and discharging processes. (b and c) Ex situ XPS spectra of NHVO-M_x: (b) V 2p and (c) O 1s. (d) SEM images of the cathode during the charging and discharging processes.

To explore the electronic structure changes during Zn²⁺ storage, ex situ XPS was used to observe the variations in the NHVO-M_x cathode during cycling. The V 2p XPS spectra revealed a two-electron reduction from V⁵⁺ to V³⁺ during discharge and a reversible oxidation during charge, which contributed to the high energy density of NHVO-M_x (Fig. 5b). Importantly, the intensity of the crystallized water peak in the O 1s spectrum increased in the fully discharged state and decreased in the fully charged state, suggesting that crystallized water enters the interlayer during cycling (Fig. 5c). The interlayer distance of approximately 12.3 Å allowed hydrated zinc ions to diffuse directly between layers, weakening their interaction with the framework and facilitating the reversible insertion and extraction of zinc ions. Thus, NHVO-M_x demonstrated excellent reversible charge–discharge performance. SEM images showed no significant morphological changes during the charge–discharge process, confirming its cycling stability, and EDS provided additional evidence for the uniform insertion of zinc ions during the cycling process (Fig. 5d and Fig. S21, SI).

Environmental and economic assessments

To highlight the significance of the short-process synthesis of high-performance vanadium-based cathodes from vanadium slag leachate, a multi-objective assessment was conducted, including comprehensive environmental, carbon emission, and economic assessments.⁴⁷ This study defines the functional unit as the production of 1 kg of doped vanadium oxide electrode material in Sichuan, China. The assessment covers the entire production process, from the raw materials (alkaline leachate of vanadium slag) to the synthesis of cathode active materials (doped vanadium oxide electrode materials) for aqueous zinc batteries. The short-process synthesis method and two traditional production processes were evaluated from environmental and economic perspectives. The two conventional methods are the multi-step precipitation and extraction techniques, followed by hydrothermal synthesis with the addition of dopants (PI and PII, Fig. 6a). In contrast, our streamlined production process involves only targeted impurity removal, vanadium precipitation and low-temperature calcination (PIII, Fig. 6a). The production process is divided into three stages: pretreatment (SI), purification (SII), and product production (SIII). The system boundaries are illustrated in Fig. S22–24 (SI), while the material and energy flows are presented in Fig. S25–27 and Tables S7–9 (SI). A detailed description of the assessment is provided in the Experimental section (SI).

Fig. 6 Multi-objective assessment of the green low-carbon strategy. (a) System boundaries of the low-carbon short process strategy for preparing doped vanadium oxide materials and two traditional strategies. (b) Comprehensive environmental assessment. (c) Carbon emission equivalents at different stages. (d) Cost comparison of different types in the three production processes. (e) Macro-economic cost comparison.

Compared to traditional strategies, the short-process approach results in minimal comprehensive environmental impact (TCEA). PI has the greatest environmental impact, primarily due to the wastewater from multiple precipitation stages. The main pollutants are ranked in the following order of impact: NH₄⁺–N > SO₄²⁻ > Cr. For the extraction stage of PII, the environmental impact of the major pollutants follows this order: Cr > COD > SO₄²⁻ > V (Fig. 6b and Tables S10–12, SI). Moreover, a reduction in carbon dioxide equivalent emissions was achieved with the short-process strategy involving doped V₂O₅, mainly due to lower electrical and thermal energy consumption associated with the purification process. As a result, the carbon dioxide equivalent emissions for the entire production process decreased significantly, from 2.45 kg for process PI and 2.83 kg for process PII to 0.81 kg (Fig. 6c). Therefore, the short-process method using leachate is considered a more environmentally friendly and carbon-reducing approach for synthesizing doping-modified vanadium-based cathodes.

Economic evaluation is a crucial aspect of assessing production processes. Cost analysis revealed that the short-process method results in cost reductions in raw material, pretreatment, and purification stages, particularly due to the elimination of complex impurity removal (Fig. S28, SI). Compared to the traditional route, the short-process approach approximately cuts energy costs by 70%, water costs by 60%, materials costs by 80%, and auxiliary costs by 65% (Fig. 6d and Fig. S29, SI). On a macro scale, reducing material losses lowers the need for initial leachate, which leads to an approximately 15% reduction in raw materials costs. Operational costs (materials, energy, and water) are reduced by about 60%, resulting in a reduction of about 40% in total costs compared to traditional methods, reaching only 51.83 CNY per kg (Fig. 6e). In summary, the short-process method utilizing vanadium slag leachate offers a simpler and more efficient synthesis pathway with reduced material and energy inputs. This not only cuts costs but also lowers pollution and carbon emissions, significantly minimizing environmental impact. This strategy aims to provide valuable insights for the development of sustainable next-generation battery materials.

Conclusions

In summary, we developed a low-carbon, short-process method for preparing NH₄⁺-intercalated and multi-element-doped vanadium oxides from vanadium slag leachate as high-performance cathodes for AZIBs. By targeting impurity removal, beneficial impurities are used as dopants, while process steps are minimized. Additionally, a low-temperature, oxygen-free calcination reduces carbon emissions and preserves essential NH₄⁺ intercalation. This synergy enhances the crystal and electronic structures: NH₄⁺ expands the interlayer spacing to facilitate Zn²⁺ ion diffusion, while both NH₄⁺ intercalation and multi-element doping narrow the band gap, improving interface charge transfer. As a result, the NHVO-M_x material exhibits excellent zinc storage performance, with a high capacity of 454.4 mAh g⁻¹ at 0.1 A g⁻¹ and exceptional cycling stability over 3000 cycles at 8 A g⁻¹. A multi-objective assessment shows that this short-process strategy not only delivers superior performance but also minimizes environmental impact, reduces carbon emissions, and lowers costs. Moreover, recycling vanadium from waste promotes sustainable metal resource utilization and advances cleaner, high-performance materials for AZIBs.

Author contributions

L. G., P. N., and H. C. conceptualized the research. L. G. conducted the experiments and wrote the initial manuscript. W. J., G. X., and J. Z. helped design certain experimental methods and offered suggestions on the manuscript. L. G., P. N., and H. C. played key roles in analysing and editing the paper. All authors reviewed the results and contributed to the work.

Conflicts of interest

The authors declare no conflict of interest.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). The supplementary information provides detailed experimental procedures, including the reagents and parameters used. It also contains the data sources and comprehensive characterization data. Supplementary information is available. See DOI: https://doi.org/10.1039/d5eb00146c.

Acknowledgements

This work was supported by the National Key R&D Program of China, Grant No. 2023YFC2908304; and the 14th Five-year Informatization Plan of Chinese Academy of Sciences, Construction of Scientific Data Centre System, Grant No. WX145XQ07-12.

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