Formamide-engineered VOPO₄ cathodes with high volumetric capacity and mass loading for aqueous zinc-ion batteries

Abstract

Aqueous zinc-ion batteries (AZIBs) have emerged as promising candidates for next-generation energy storage systems due to their inherent safety, cost-effectiveness, and high theoretical capacity. However, their practical application remains constrained by limited cycling stability and sluggish ion diffusion kinetics, particularly under high mass loading conditions. These limitations are primarily attributed to the restricted ion transport pathways within the electrode structure and structural degradation caused by repeated zinc-ion insertion and extraction in highly loaded electrodes. To address these challenges, formamide (FA)-inserted VOPO₄ (FA-VOPO₄) nanosheet cathodes were designed with expanded interlayer spacing (9.3 Å), where FA molecules partially replace interlayer water, thereby enhancing both structural stability and ion transport pathways. This unique structural modification, supported by synergistic hydrogen bonding between FA and residual water, significantly improves Zn²⁺ diffusion kinetics and charge transfer properties, as confirmed by electrochemical tests and theoretical analysis. Consequently, FA-VOPO₄ electrodes delivered a remarkable volumetric capacity of 733 mAh/cm³ at 40 mA/g, approximately 8 times higher than that of the VOPO₄·2H₂O electrode, and retained 82.1% of their capacity after 1000 cycles at 1 A/g with a mass loading of 10 mg/cm². Even at a high mass loading of 20 mg/cm² (4.4 mAh/cm²), the FA-VOPO₄ cathode maintained a volumetric capacity of 535 mAh/cm³. These findings provide valuable insights into electrode design strategies for high-performance AZIBs, contributing to the development of safer, more efficient energy storage technologies with potential applications in grid storage and portable electronics.

Graphical abstract

Keywords

high mass loading / aqueous zinc-ion battery / high volumetric capacity / FA-VOPO₄

1 Introduction

Aqueous zinc-ion batteries (AZIBs) are gaining attention as promising energy storage solutions for grid applications and flexible wearable devices due to zinc’s favorable properties [1], including a low redox potential (−0.763 V versus standard hydrogen electrode), high theoretical volumetric capacity (5851 mAh/cm³), cost-effectiveness, environmental compatibility, and intrinsic safety [2–5]. Paired with suitable cathode materials, AZIBs can achieve competitive energy densities. However, existing cathode materials, such as manganese-based compounds [6,7], Prussian blue analogs [8,9], and vanadium-based compounds [10,11], face substantial challenges, including structural degradation, capacity fading, and poor cycling stability. These limitations are further exacerbated under high mass loading conditions, which are essential for improving energy density but hindered by low conductivity, sluggish reaction kinetics, and poor interfacial stability with aqueous electrolytes [3,12,13]. Furthermore, the limited specific capacity of cathode materials under high mass loading reduces the overall volumetric energy storage potential, preventing full utilization of the high volumetric capacity of the zinc anode.

Achieving high-performance cathodes under high mass loading conditions requires a combination of structural robustness, high electronic and ionic conductivity, and fast reaction kinetics. Ideal cathode materials should enable efficient zinc-ion insertion and extraction while maintaining structural integrity throughout extended cycling [14–17]. Layered vanadium-based compounds, such as VOPO₄, stand out due to their open frameworks and tunable interlayer spacing, which facilitate rapid ion transport [10,18–21]. Among these, VOPO₄ has emerged as a promising candidate due to its high theoretical capacity (over 300 mAh/g) and superior reaction kinetics [22–25]. However, its practical application in AZIBs is impeded by several critical challenges: the high charge-to-size ratio of zinc ions leads to significant electrostatic repulsion, causing lattice expansion and structural degradation [26,27]. Additionally, VOPO₄ tends to irreversibly transform into less active phases, such as VO_x, resulting in rapid capacity decay and reduced cycle life [28–30].

To address these challenges, various interlayer engineering strategies have been explored, including expanding interlayer spacing via self-assembled organic molecules, to enhance the electrochemical performance of VOPO₄ cathodes in zinc-ion batteries [22,31,32]. Additionally, interlayer cation pre-intercalation (Li⁺, Na⁺, K⁺, NH₄⁺, Cu²⁺, etc.) has also been employed to enlarge interlayer distance and reduce electrostatic interactions between inserted Zn²⁺ and the crystal framework, significantly improving Zn²⁺ diffusion kinetics [33–37]. Another approach involves introducing water molecules to form VOPO₄·2H₂O, where interlayer water molecules alleviate electrostatic repulsion and mitigate volumetric changes during cycling, thereby enhancing capacity and stability [38]. However, under high mass loading conditions, this approach has proven insufficient due to sluggish ion diffusion and limited electronic conductivity. The ion diffusion pathways may lack the speed and interconnectivity to accommodate the large flux of Zn²⁺ ions required under high mass loading. This results in sluggish ion diffusion kinetics, particularly in thick electrodes, where deeper layers become less accessible [39]. While water molecules increase interlayer spacing to facilitate zinc-ion insertion and extraction, they also strongly hydrate the zinc ions, reducing their mobility and slowing ion diffusion kinetics [40–42]. This issue becomes particularly pronounced in thick electrodes, ultimately limiting the electrochemical performance of VOPO₄·2H₂O.

In this study, a novel strategy is proposed to overcome these limitations by intercalating formamide (FA) instead of water into the VOPO₄ interlayer. FA shares many physical and molecular-structural properties with water but offers high donor number (DN), which quantifies the solvent’s ability to donate electron pairs or act as a Lewis base (refer to Table S1 for details). This property enables FA to establish moderate interactions with Zn²⁺ ions while enhancing their mobility. The intercalation of FA increases the interlayer spacing of VOPO₄, facilitating easier zinc-ion insertion and extraction, while also improving ionic and electronic conductivity, thus significantly enhancing redox kinetics. Furthermore, synergistic hydrogen bonding between FA and residual water molecules stabilizes the layered structure [43,44], ensuring superior cycling stability.

The resulting FA-intercalated VOPO₄ material exhibits exceptional electrochemical performance. At a mass loading of 7 mg/cm², FA-VOPO₄ delivers a remarkable specific mass capacity of 463 mAh/g and a volumetric capacity of 733 mAh/cm³, approximately 8 times greater than that of VOPO₄·2H₂O electrode, outperforming similar results reported in the literature. Even at a significantly high mass loading of 20 mg/cm² (4.4 mAh/cm²), the FA-VOPO₄ material exhibits a volumetric capacity of 535 mAh/cm³. After 1000 cycles at a current density of 1 A/g and a mass loading of 10 mg/cm², the FA-VOPO₄ electrode retains 82.1% of its capacity (564 mAh/cm³). This work represents a crucial step toward the development of high-energy-density AZIBs for practical applications.

2 Results and discussion

FA-VOPO₄ was synthesized the in situ intercalation of FA into VOPO₄·2H₂O through a combined hydrothermal and ultrasonic treatment, as detailed in the Electronic Supplementary Material. The structural characteristics of FA-VOPO₄ were investigated using a comprehensive suite of techniques. The X-ray diffraction (XRD) pattern (Fig.1(a)) reveals a shift in the (001) diffraction peak from 11.9° to 9.4° after FA insertion, indicating an expansion of the interlayer spacing from 7.4 to 9.3 Å. This increased interlayer spacing is expected to enhance Zn²⁺ migration kinetics [45]. Fourier transform infrared (FTIR) spectra of VOPO₄·2H₂O and FA-VOPO₄ (Fig.1(b)) confirm the presence of FA within the structure, as evidenced by characteristic peaks at 3320 cm⁻¹ (N–H stretching), 1673 cm⁻¹ (C=O stretching), 1416 cm⁻¹ (C–N stretching), and 1616 cm⁻¹ (O–H bending), corresponding to FA and residual H₂O molecules [46,47]. X-ray photoelectron spectroscopy (XPS) N 1s spectra (Fig. S1) further support the partial substitution of water by FA [48]. The O–H bending vibration of residual water shifts from 1616 cm⁻¹ in VOPO₄·2H₂O to 1604 cm⁻¹ in FA-VOPO₄, indicating that interlayer water forms stronger hydrogen bonds with the C=O or N−H groups of FA, resulting in a decreased O−H bond vibrational frequency [43,44]. Solid-state ¹³C nuclear magnetic resonance (NMR) spectra (Fig.1(c)) show a shift in the C(N)=O peak from 166.2 ppm in pure FA to 167.8 ppm in FA-VOPO₄, attributed to hydrogen bonding interactions between FA and residual water molecules.

Microscopic morphology characterized using scanning electron microscopy (SEM) (Figs. 1(d)–1(f)), reveals that the FA-VOPO₄ nanosheets (~104 nm) are thinner than those of VOPO₄·2H₂O (~552 nm) (Fig. S2). High-resolution transmission electron microscopy (HRTEM) images (Fig. S3) display clear lattice fringes corresponding to the (001) and (200) crystal planes, consistent with XRD results. Energy dispersive X-ray spectroscopy (EDS) mapping (Fig. S4) confirms the uniform distribution of carbon, nitrogen, oxygen, vanadium, and phosphorus throughout the nanosheets, indicating homogeneous incorporation of FA within the structural framework.

The expanded interlayer spacing in FA-VOPO₄ facilitates rapid zinc-ion insertion and extraction, even under high mass loading conditions. As shown in Fig.2(a) and S5, the FA-VOPO₄ cathode with a mass loading of 7 mg/cm², delivers a high specific capacity of 463 mAh/g (733 mAh/cm³) at 0.02 A/g, along with excellent rate capability, maintaining a specific capacity of 386 mAh/g at 0.4 A/g. This performance significantly surpasses that of VOPO₄·2H₂O, which exhibits only 85 mAh/g (83 mAh/cm³) at 0.02 A/g, approximately one-eighth the volumetric capacity of FA-VOPO₄ (refer to Fig. S6 for details). As the FA-VOPO₄ mass loading increases from ~4 mg/cm² to ~20 mg/cm², the areal capacity rises from 1.3 to 4.4 mAh/cm², while the volumetric capacity improves from 403 to 535 mAh/cm³ (Fig.2(b), S7 and S11; volumetric capacity calculation detailed in Fig. S8), meeting commercial benchmarks of 3–5 mAh/cm² while maintaining excellent volumetric energy density retention.

The observed decrease in specific capacity at higher loadings is attributed to the increased electron and ion transport distances in thicker electrodes. Notably, the FA-VOPO₄ cathode with a mass loading of 10 mg/cm² exhibits outstanding long-term cycling performance, retaining a volumetric capacity of 478 mAh/cm³ after 1000 cycles at 1 A/g, with an average Coulombic efficiency (CE) of 99.95% (Fig.2(e)). In stark contrast, VOPO₄·2H₂O undergoes rapid degradation, retaining only 31.9% of its initial capacity after 784 cycles under the same conditions. Remarkably, FA-VOPO₄ also demonstrates robust cycling stability under various rates and high mass loadings, retaining 85.6% of its initial capacity after 600 cycles at 0.5 A/g and 82.1% after 2000 cycles at 2 A/g, with a high mass loading of 10 mg/cm² (Figs. S9 and S10). Furthermore, even at an exceptionally high mass loading of 19.78 mg/cm² (Fig. S11), FA-VOPO₄ maintains remarkable cycling stability, outperforming other cathode materials reported in the literature (Table S2).

In this study, the VOPO₄ cathode material exhibited a gradual increase in capacity during electrochemical charge/discharge cycling. To investigate the underlying reaction mechanism, in-depth electrochemical experiments and analyses were conducted. Differential capacitance (dQ/dV) plots (Fig.3(a) and Fig.3(b), S12(b) and S12(c)) indicate similar electrochemical behavior for both VOPO₄ materials during cycling. Four pairs of redox peaks were initially observed at 1.4/1.6, 1.1/1.3, 0.8/1.1, and 0.5/0.7 V. As cycling progressed, the redox peaks at 1.4/1.6 V and 1.1/1.3 V gradually diminished, while those at 0.5/0.7 and 0.8/1.1 V became more pronounced. This evolution is attributed to the progressive dissolution of PO₄³⁻ and the structural transformation of VOPO₄ into vanadium oxides (VO_x) [28,29].

Unlike VOPO₄·2H₂O, which exhibits a steady decline in electrochemical activity characterized by the fading of all redox peaks, FA-VOPO₄ retains high electrochemical activity even after transitioning into VO_x. Following this transformation, the FA-VOPO₄ cathode achieved a capacity of 303 mAh/g, significantly surpassing VOPO₄·2H₂O, which retained only 42 mAh/g (Fig.3(b)).

The phase transformation of FA-VOPO₄ and VOPO₄·2H₂O during cycling was systematically characterized using XRD and EDS. As shown in Fig. S13(a), the initial P/V atomic ratios of pristine VOPO₄·2H₂O and FA-VOPO₄ electrodes were 0.92 and 0.88, respectively. After 100 cycles in 2 mol/L Zn(OTf)₂ electrolyte, these ratios decreased to 0.05 (VOPO₄·2H₂O) and 0.03 (FA-VOPO₄), indicating significant structural decomposition. XRD patterns (Fig. S13(b)) confirm the formation of V₂O₅·H₂O in both materials, consistent with previous reports describing the conversion of VOPO₄ into vanadium oxides during cycling. Additionally, surface characterization of the cycled electrodes (Fig. S13(b)) revealed minor formation of Zn₃(PO₄)₂(H₂O)₄, attributed to surface phosphate-zinc interactions, while analyzing V₂O₅·H₂O remained the dominant phase (see Fig. S13(b) and S13(c) for detailed depth profiling and EDS results).

To further elucidate the ion storage mechanism in FA-VOPO₄, ex-situ XRD and XPS analyses were performed. As shown in Fig.3(c) and Fig.3(d), the (200) diffraction peak of FA-VOPO₄ shifted to lower angles (28–30°) during discharge, indicating interlayer expansion due to H⁺/Zn²⁺ insertion [49]. Upon charging, the peak returned to its original position, demonstrating excellent structural reversibility. This observation was further corroborated by ex-situ XPS analysis (Fig.3(e)), which showed a reversible change in the vanadium shifted valence state from mixed V⁴⁺/V⁵⁺ to V³⁺/V⁴⁺ during discharge due to H⁺/Zn²⁺ intercalation. Upon recharging, the valence state reverted to its initial configuration. Simultaneously, Zn 2p spectra (Fig.3(f)) showed the reversible appearance and disappearance of zinc ion signals, confirming the reversible Zn²⁺ insertion/extraction process.

The ion storage characteristics in FA-VOPO₄ electrodes were investigated through dynamic analysis using cyclic voltammetry (CV) at scan rates ranging from 0.2 to 1.0 mV/s. The FA-VOPO₄ cathode exhibited similar CV curve profiles across varying scan rates, indicating minimal polarization voltage (Fig.4(a)). As the scan rate increased, the area enclosed by the CV curves expanded progressively, suggesting enhanced capacitive behavior. Electrochemical kinetics were evaluated using

(1)i=a{\nu }^b,

where i represents the peak current (mA), \nu is the scan rate (mV/s), a and b are fitting parameters. A b-value of 1 corresponds to a surface-controlled capacitive process, while a value near 0.5 suggests a diffusion-controlled behavior typical of faradaic reactions [50,51]. The calculated b-values for redox peaks 1 to 6 were 0.81, 0.85, 0.87, 0.98, 0.99, and 1.05, respectively (Fig.4(b)), indicating that charge storage in FA-VOPO₄ is predominantly governed by capacitive behavior [52].

Ion diffusion kinetics were further explored using the galvanostatic intermittent titration technique (GITT). The Zn²⁺ diffusion coefficients (D_Zn²⁺) for FA-VOPO₄ during both charging and discharging processes ranged from approximately 10⁻⁸ to 10⁻⁹ cm²/s, significantly higher than those of VOPO₄·2H₂O, which ranged from 10⁻⁹ to 10⁻¹⁰ cm²/s (Fig.4(c)). This improvement is attributed to the FA intercalation, which improves ion transport pathways. Electrochemical impedance spectroscopy (EIS) further revealed a significantly lower charge-transfer resistance (R_ct) for FA-VOPO₄ compared to VOPO₄·2H₂O, indicating superior intrinsic electronic conductivity (Fig.4(d); refer to Fig. S14 for details of the equivalent circuit model).

To understand the origin of the improved ion diffusion, density functional theory (DFT) calculations were performed. Zn²⁺ migration pathways were modeled using the climbing image nudged elastic band (CI-NEB) method to calculate corresponding diffusion energy barriers (Fig.4(e)–Fig.4(i) and S15). Intercalation of FA into VOPO₄·2H₂O induces a significant expansion of the interlayer spacing. In pristine VOPO₄·2H₂O, the relatively narrow interlayer channels restrict Zn²⁺ mobility, where oxygen atoms from phosphate groups and interlayer water molecules create a complex electrostatic environment. These strong interactions with Zn²⁺ result in a high diffusion energy barrier of 0.60 eV [22,31]. The intercalation of FA into VOPO₄·2H₂O disrupts the original interlayer electrostatic field. Hydrogen bonding between FA’s C=O or N–H groups and interlayer water molecules, coupled with expanded interlayer spacing, effectively alleviates electrostatic confinement, resulting in a significantly lower Zn²⁺ diffusion of 0.38 eV, markedly lower than the 0.60 eV observed in pristine VOPO₄.

Furthermore, FA-VOPO₄ demonstrates enhanced Zn²⁺ diffusion kinetics and improved electronic conductivity, supporting its excellent electrochemical performance even under high mass loading conditions.

3 Conclusions

In summary, FA-VOPO₄ nanosheets with enlarged interlayer spacing (9.3 Å) were successfully synthesized via a straightforward ultrasonic pulverization method. Structural characterization confirmed that FA partially replaced interlayer water molecules, leading to expanded interlayer spacing and the formation of additional Zn²⁺ migration pathways. Moreover, synergistic hydrogen bonding between FA and residual water molecules further stabilized the layered structure, contributing to excellent cyclic stability. Theoretical calculations revealed that FA intercalation significantly reduced the Zn²⁺ diffusion energy barrier, while electrochemical analysis demonstrated enhanced Zn²⁺ diffusion kinetics and enhanced electronic and ionic conductivity.

As a result, the FA-VOPO₄ electrode exhibited outstanding electrochemical performance, even under high mass loading conditions. Specifically, at a mass loading of 7 mg/cm² and a current density of 40 mA/g, it achieved a remarkable specific capacity of 463 mAh/g and a volumetric capacity of 733 mAh/cm³. Even at a mass loading of 20 mg/cm², it maintained a capacity of 535 mAh/cm³. Long-term cycling performance further demonstrated excellent durability, with the electrode retaining 82.1% of its capacity (564 mAh/cm³) after 1000 cycles at 1 A/g and a mass loading of 10 mg/cm². This study presents a significant advancement in zinc-ion battery development, offering a promising pathway toward high-energy-density, safer, and durable aqueous energy storage systems.

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