Structural modulation of NH₄V₄O₁₀ cathode materials for low-temperature zinc-ion energy-storage devices

Abstract

With the characteristics of nontoxicity and environmental benignity, aqueous zinc ion batteries (AZIBs) are rapidly emerging as potential competitors for high-performance energy-storage systems. Nevertheless, several issues hinder their further development, such as the sluggish electrochemical activity and inevitable dissolution of cathode materials. In this work, we introduced oxygen defects (O_d) into the NH₄V₄O₁₀ lattice, which facilitated the transport velocity of Zn²⁺ ions and enhanced their electrical conductivity. Zn//NHVO-O_d-1 batteries showed a reversible capacity of 475.3 mA h g⁻¹ at 0.2 A g⁻¹. At low temperature (0 °C), the cells also demonstrated a capacity retention of 100% after 1000 cycles at 1.0 A g⁻¹. Assembled soft-package devices presented favorable mechanical resilience at different bending conditions.

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1. Introduction

The excessive environmental pollution and increasing greenhouse effect have stimulated the exploration of renewable energy-storage technologies.^1–3 Recently, lithium-ion batteries (LIBs) have dominated the portable accessory and grid-scale energy-storage markets.^4–6 However, their further application and commercialization are severely hampered by thermal runaway issues and the scarcity of lithium.^7,8 AZIBs have become promising devices to replace LIBs with outstanding safety, abundant raw materials and high theoretical capacity.^9,10 Therefore, it is imperative to design cathode materials that are compatible with the zinc anode. In previous reports, cathodes investigated for AZIBs include organic compounds, Prussian blue analogs (PBAs), manganese oxides and vanadium-based materials.^11–14 However, manganese oxides undergo structural changes during the operating process, which can lead to rapid performance decay.¹⁵ PBAs release toxic substances and harmful gases during operation.¹⁶ Organic compounds present the intrinsic drawback of low electrical conductivity.¹⁷ Owing to the various crystal structures of vanadium (V⁵⁺ ↔ V⁴⁺ ↔ V³⁺) and superior safety of neutral or weakly acidic electrolytes, vanadium-based materials have attracted interest as potential cathode materials for AZIBs.¹⁸ Additionally, the introduction of defects can effectively accelerate ion-diffusion kinetics and increase the discharge capacity of the cells. Therefore, it is a widely used strategy for the modification of cathode materials.¹⁹

Ammonium vanadate (NH₄V₄O₁₀) materials possess an excellent Zn²⁺-storage ability owing to the bilayer structure of V₄O₁₀.^20,21 Besides, NH₄⁺ ions act as “pillars” to maintain the interlayer structure against the volumetric change that occurs during charging and discharging.^22,23 Zong et al. synthesized deficient NH₄V₄O₁₀ nanosheets via a heat-treatment route.²⁴ Assembled batteries showed a reversible capacity of 457.0 mA h g⁻¹ at 0.1 A g⁻¹. Moreover, Wang and coworkers introduced Al³⁺ into ammonium vanadate materials using a hydrothermal approach.²⁵ The assembled device showed a cycling capacity of 475.8 mA h g⁻¹ at 0.2 A g⁻¹. Liu et al. increased the interlayer spacing of NH₄V₄O₁₀ by compositing it with rGO.²⁶ The resulting cathodes presented a capacity of 551.0 mA h g⁻¹ at 0.1 A g⁻¹. Oxygen defects (O_d) are vacancy defects formed by the loss of oxygen ions from the crystal lattice. Their appearance leads excess electrons to form defect sites, which create delocalized electron clouds to improve electronic conductivity.²⁷ However, there has been a lack of exploration of oxygen-deficient vanadate as a cathode material for operating under severe working conditions.

In this work, we introduce oxygen defects into the lattice of NH₄V₄O₁₀ nanobelts by adding thiourea. The obtained products were denoted as NHVO-O_d. Thiourea was added as it can tailor the microscopic morphology and internal structural defects of materials, which can ultimately improve the reaction kinetics. An assembled Zn//NHVO-O_d-1 battery showed an initial specific capacity of 475.3 mA h g⁻¹ at 0.2 A g⁻¹. Meanwhile, it could retain 90.8% of its initial capacity at 5.0 A g⁻¹ for 1000 times cycling. At low temperature (0 °C), the assembled button cells achieved a cycling life of 1000 times with a capacity retention of 93.9% at 5.0 A g⁻¹. Furthermore, soft-package devices were assembled to investigate their mechanical stability, demonstrating a capacity retention of 66.7% at 1.0 A g⁻¹ after 600 times charging and discharging. The synergetic utilization of the PAM hydrogel and NHVO-O_d-1 materials endow the flexible batteries with favorable mechanical toughness and cycling stability at 0 °C.

2. Experimental section

2.1 Material synthesis

First, 4 mmol NH₄VO₃ and 4 mmol H₂C₂O₄·2H₂O were dissolved in 40 mL DI and magnetically stirred for 30 min at 70 °C. Then, 1 or 2 mmol thiourea was added into the above mixture, respectively. After cooling, 4 drops of 3 M hydrochloric acid was added to adjust the pH value of the mixtures to 2. Subsequently, each mixture was transferred into 100 mL PTFE-lined autoclave and kept at 180 °C for 24 h. After cooling, the precursors were sequentially centrifuged three times alternately with anhydrous ethanol and DI. Finally, the resulting products were dried in a vacuum oven for 9 h at 60 °C. The collected samples were named as NHVO-O_d-1 and NHVO-O_d-2, respectively. For comparison, pure NH₄V₄O₁₀ samples were also synthesized without adding thiourea.

To prepare the hydrogel electrolyte (PAM), 5 g of acrylamide (AM) was dissolved into 15 mL of 3 M Zn(CF₃SO₃)₂ electrolyte. After that, 20 mg of (NH₄)₂S₂O₈ and 3 mg of N,N-methylenebisacrylamide were subsequently added into the above mixture. After stirring, the solution was poured into a medical syringe and injected into a fixed plastic mold. The molds were dried in a drying oven at 60 °C for 3 h to obtain the PAM hydrogel.

2.2 Structure and morphology characterization

The crystal structures of the samples were characterized by X-ray diffraction (XRD, Rigaku Ultima IV, Cu Kα radiation, λ = 0.1541 nm, 40 KV). Moreover, we employed X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-α) to evaluate the elemental composition of the samples. The O_d was tested through electron paramagnetic resonance spectroscopy (EPR, Bruker EMXplus-6/1). Finally, the microstructure of the samples was observed by scanning electron microscopy (SEM, Gemini, 300-71-31).

2.3 Electrochemical characterization

Cathode materials were prepared by evenly combining polyvinylidene fluoride (PVDF), carbon black (Super P), and the as-fabricated samples with a mass ratio of 1 : 2 : 7. Then an appropriate amount of N-methyl-l-2-pyrrolidone (NMP) was added to form a slurry, which was coated on carbon paper. The active materials showed an average loading mass of 1.4 mg g⁻¹. Then, the samples were placed in a vacuum drying oven at 60 °C overnight. A series of coin cells were assembled with the prepared materials, zinc foils, 3 M Zn(CF₃SO₃)₂ electrolyte, and glass fiber separators.

For flexible devices, the cathode and anode were separated by polyacrylamide (PAM) gel electrolyte. They were encapsulated with a layer of aluminum–plastic film. A temperature test chamber (Neware, MGDW-225-40C) was employed to study the cycling performance of the devices at 0 °C. An automatic battery test system (Neware, CT-4008T) was utilized to evaluate the galvanostatic charge/discharge (GCD) curves. Electrochemical impedance spectra (EIS) and cyclic voltammetry (CV) curves were determined using an electrochemical workstation (Shanghai Chenhua, CHI660E) within the potential range of 0.3–1.5 V.

3. Results and discussion

XRD was employed to study the crystal structure of the samples. As shown in Fig. 1a, the diffraction peaks of the products matched with the NH₄V₄O₁₀ phase (JCPDS no. 31-0075). The main characteristic peaks located at 9.2°, 25.5°, 27.68°, 33.93°, 47.05°, and 49.79° corresponded to the (001), (110), (111), (−311), (−205), and (020) crystal planes, respectively. Besides, the (001) peaks of the NHVO-O_d-1 and NHVO-O_d-2 samples were slightly shifted to the left due to the increase in O_d after adding thiourea.²⁸ During hydrothermal reactions, thiourea decomposes to form hydrogen sulfide (H₂S), carbon dioxide (CO₂), and ammonia (NH₃). H₂S gas reduces some vanadium elements of NH₄V₄O₁₀ to introduce oxygen defects in products.²⁹ Moreover, the broad peaks and rough baselines of the NHVO-O_d-2 pattern demonstrated the inferior crystallinity of the products, which caused a degradation of the reversible capacity.

Fig. 1 Structural characterization: (a) XRD patterns; (b) XPS survey spectra, (c) O 1s, (d) V 2p, and (e) N 1s; and (f) EPR spectra of the NH₄V₄O₁₀ and NHVO-O_d-1 samples.

Next, we employed XPS to study the elemental valence distribution and chemical composition of the materials. Fig. 1b shows the full spectrum of the NHVO-O_d-1 sample, which indicated the presence of O, V, N, and C elements. Besides, no signal peak of S was found in the spectrum. The O 1s spectra of both materials are presented in Fig. 1c. The primary peaks were de-convoluted into several peaks located at 531.48, 530.58, and 529.98 eV, which belonged to chemisorbed oxygen (O–H), oxygen defects (O_d), and lattice oxygen (V–O), respectively.³⁰ With the adding of thiourea, the ratio of oxygen defects increased from 25.6% to 27.4%, as shown in the inset. Furthermore, the presence of defects could be confirmed in V 2p spectra (Fig. 1d); whereby the signal peaks located at 517.58/524.88 and 516.18/523.58 eV corresponded to V⁴⁺ and V⁵⁺, respectively.³¹ By integrating the areas of the signal peaks, the calculated V⁴⁺/V⁵⁺ ratio of NHVO-O_d-1 (0.64) was found to be higher than that of NH₄V₄O₁₀ (0.43). This suggests that some V⁵⁺ ions were reduced to V⁴⁺, which was attributed to the many oxygen defects formed.³² The N 1s peaks were fitted to two peaks, including neutral nitrogen (–NH–) at 401.58 eV and positively charged nitrogen (–NH⁺–) at 399.78 eV, as shown in Fig. 1e.³³ Moreover, EPR was employed to probe the concentration of oxygen defects in the products. Fig. 1f presents the spectra of the two samples with sharp and strong symmetric peaks at g = 2.004, demonstrating the consistence of oxygen defects.³⁴ The higher signal intensity of the NHVO-O_d-1 samples suggested more defects were formed, which coincided with the results in the O 1s and V 2p spectra.

Subsequently, we observed the morphology of the materials. From the SEM images, it could be seen that the NH₄V₄O₁₀ samples possessed large agglomerates consisting of many nanoparticles (Fig. 2a and b). However, the NHVO-O_d-1 samples presented nanobelt-like shapes, which could be explained by the spontaneous nucleation and aggregation of nanorods (Fig. 2c). The average thickness of the nanobelts was 80 nm, as described in Fig. 2d. When the addition of thiourea reached 2 mmol, the nanobelts broke and manifested a rod-shaped morphology with different lengths (Fig. 2e and f). Thus, the morphology of the materials was regulated by the amount of added thiourea.

Fig. 2 Morphology characterization: (a) and (b) SEM images of the NH₄V₄O₁₀, (c) and (d) NHVO-O_d-1, and (e) and (f) NHVO-O_d-2 samples.

To determine the electrochemical performance of the samples, detailed characterizations were performed in the potential range of 0.3–1.5 V. Fig. 3a displays the CV curves of all the devices obtained at a scan rate of 0.2 mV s⁻¹. Thereinto, the Zn//NHVO-O_d-1 battery manifested a large integrated area, revealing its high discharging capacity. Two pairs of reduction/oxidation peaks were located at 0.57/0.93 V and 0.66/1.01 V, respectively, which were in accordance with the insertion and de-insertion process of Zn²⁺.³⁵ Fig. 3b shows the specific capacities of all the samples at 0.2 A g⁻¹. The reversible capacities of the NH₄V₄O₁₀, NHVO-O_d-1, and NHVO-O_d-2 cathodes were 368.1, 475.3, and 404.6 mA h g⁻¹ at 0.2 A g⁻¹, respectively. After 80 times cycling, the Zn//NHVO-O_d-1 devices retained a capacity of 414.7 mA h g⁻¹ with a retention rate of 87.3%. From Fig. 3c, it could be seen that several inclined platforms appeared in the GCD curves, which were consistent with the corresponding CV curves. This demonstrates that the de-intercalation and intercalation of Zn²⁺ in the reaction were accompanied by the valence transition of V.³⁶

Fig. 3 Electrochemical performances. (a) CV curves at 0.2 mV s⁻¹. (b) Cycling performance at 0.2 A g⁻¹. (c) GCD curves at 0.2 A g⁻¹. (d) Long-term cycling performance at 5.0 A g⁻¹. (e) Rate capability and (f) GCD curves at various current densities. (g) Rate capability. (h) Long-term cycles. (i) GCD curves at 0 °C.

The charge/discharge multiplier performance is a significant parameter to study the comprehensive performance of batteries. As indicated in Fig. 3d, the devices delivered capacities of 255.1, 365.4, and 329.3 mA h g⁻¹ at a current density of 5.0 A g⁻¹, respectively. After 1000 times cycling, the reversible capacities of the NHVO-O_d-1 cathodes was still higher than those of the electrodes and they maintained a retention rate of 90.8%. The incorporation of oxygen defects accelerated the migration rate of Zn²⁺, which effectively improved the specific capacity and cycling stability. From Fig. 3e, it could be seen that the Zn//NHVO-O_d-1 batteries possessed specific capacities of 455.9, 438.1, 426.9, 406.6, 392.6, and 360.7 mA h g⁻¹ at 0.2, 0.5, 1.0, 2.0, 3.0, and 5.0 A g⁻¹, respectively. This reveals that the batteries possessed excellent performance under variable operating conditions. When the current density recovered to 0.2 A g⁻¹, they could still keep a capacity of 452.4 mA h g⁻¹. Compared with the other two cells, the NHVO-O_d-1 electrodes delivered superior rate performances. This suggests that the oxygen defects in the composite cathodes accelerate the migration rate of Zn²⁺ and enhance the electrical conductivity. The induction of oxygen defects produces more active sites than in the original material, which then allows accommodating extra reversible Zn²⁺ intercalation. However, structural collapse owing to excessive oxygen defects adversely affects the capacity of the Zn//NHVO-O_d-2 devices. As shown in Fig. 3f, the reversible capacity gradually reduced as the current density increased. We further studied the discharge capacities of the Zn//NHVO-O_d-1 batteries at low temperature (0 °C). The devices delivered specific capacities of 352.2, 324.6, 296.8, 257.3, 225.9, and 170.9 mA h g⁻¹ at 0.2 to 5.0 A g⁻¹, respectively (Fig. 3g). The presence of oxygen defects reduced the diffusion barrier and optimized the migration path of Zn²⁺, which accelerated the charge-transfer kinetics. When it recovered to 0.2 A g⁻¹, the cathodes delivered a capacity of 348.6 mA h g⁻¹. Moreover, the cells demonstrated nearly 100% capacity retention at 5.0 A g⁻¹ after 1000 times cycling (Fig. 3h). The excellent cycling stability was attributed to the stable crystal structure of the cathode material during charging and discharging. Oxygen defects alleviates the electrostatic intercalation of Zn–O bonds, which stabilizes the lattice interlayer structure.³⁷ Fig. 3i shows the corresponding GCD curves at various current densities at low temperature. As the current density increased, the curves displayed a gradual decline in specific capacity. The consistent shape of these curves underscores their excellent cycling stability under 0 °C.

Next, we evaluated the electrode reaction kinetics behavior of the devices. Fig. 4a displays the CV curves of the devices at scan rates ranging from 0.2 to 1.0 mV s⁻¹. As the sweep speed intensified, the negative and positive signal peaks were shift toward the low and high potential regions, respectively. A gradual expansion of the integration area is correlated with the polarization behavior during the operation of the battery. The relationship between the peak currents and scanning rates of CV curves can be fitted based on eqn (1) below:³⁸

i = av^b (1)

where v and i are the sweep rate and current density, respectively, and a and b are adjustable parameters. When the b value is close to 1, the redox reaction inside the electrode material is dominated by the pseudocapacitive behavior. When b is close to 0.5, the reaction process is determined by ion diffusion. When the b value ranges from 0.5 to 1, it indicates that both pseudocapacitance and battery behavior dominate the reaction process. After analyzing the slope of the fitted lines (log i vs. log v), the b values were found to be 0.91, 0.85, 0.84, and 0.79, revealing the coexistence of a battery behavior and pseudocapacitive behavior in the reaction process (Fig. 4b). Eqn (1) can be further written as follows:³⁹

i(V) = k₁v^1/2+ k₂v (2)

where k₁ and k₂ are fixed parameters, and The k₂v and k₁v^1/2 refer to surface diffusion-controlled behavior and the capacitance-controlled reaction, respectively. The capacitive contribution in Fig. 4c was calculated through eqn (2). Fig. 4c indicates that the pseudocapacitance contribution ratio increased from 88.4% to 94.3% as the sweeping speed increased during the redox processes. Next, the galvanostatic intermittent titration technique (GITT) was employed to investigate the relationship between ion diffusion and charge transfer. By recording the electrochemical response data of the pulse electrolysis mass-transfer process, the ion-diffusion performance of a battery can be evaluated. The diffusion coefficients of Zn²⁺ (D_Zn) were further explored by GITT curves:⁴⁰where τ is the relaxation time (s), ΔE_τ means the voltage change during constant current charging and discharging, S is the total contact area of the cathode plate (cm²), ν_m is the molar volume (cm³ mol⁻¹), and M_B and m_b and are the molecular mass (g mol⁻¹) and loading mass of the cathode (g) of the active material coated on the electrode sheet, respectively. In this work, the area of the cathode sheet was 0.785 cm² and the loading mass was 1.33 mg. Also, ΔE_s is the steady-state voltage change with the current pulse. The two parameters were obtained by eliminating the voltage change after the relaxation time. As shown in Fig. 4d, the D_Zn values of the Zn//NHVO-O_d-1 device were between 10⁻⁸ and 10⁻⁶ cm² s⁻¹. The results demonstrate that oxygen defects accelerated the migration of Zn²⁺ in the interlayer spacing.

Fig. 4 Electrochemical reaction kinetics of the batteries: (a) CV curves. (b) The fitting plots of log(i) versus log(v). (c) Capacitance contribution ratios at various scan rates. (d) GITT curves. (e) EIS plots. (f) Ragone plots of various vanadate materials.

Next, EIS was employed to study the resistance of the devices. The electrode reaction system corresponds to the current reaction signal at the interference of sinusoidal alternating current. From Fig. 4e, the X-axis represents the actual impedance value and the Y-axis represents the theoretical value, which were used to describe the electrochemical impedance spectrum caused by a series of sinusoidal alternating currents. The curves of the prepared sample consisted of a semicircle in the high-frequency region and a straight line in the low one. The high-frequency region in this figure is in the lower left, showing a semicircular curve that represents the charge-transfer resistance (R_ct). By fitting the curves, it could be seen that the NHVO-O_d-1 electrode delivered a lower resistance (R_ct = 48.5 Ω) compared to the NH₄V₄O₁₀ (R_ct = 85.4 Ω) and NHVO-O_d-2 (R_ct = 59.3 Ω) samples. This also confirmed that oxygen defects can enhance the electron transfer in redox reactions. The power density and energy density of the batteries can be computed by the following equations:⁴¹

P = IU/2m (4)

E = QU/2m (5)

where P (W kg⁻¹) refers to the power density, E (W h kg⁻¹) is the energy density, and I (A), U (V), Q (A h), and m (kg) represent the current density, operating voltage, discharge capacity, and the loading mass of the active substrates, respectively. As indicated in Fig. 4f, Ragone plots were obtained to allow comparison with some other reported V-based cathode materials.^42–47 The Zn//NHVO-O_d-1 battery delivered a power density of 2.25 kW kg⁻¹ and an energy density of 162.3 W h kg⁻¹, which were superior to previous reported devices.

To explore the variation in the valence and chemical composition of the elements of the NHVO-O_d-1 cathode, we subsequently studied the cathode reaction behavior through ex situ XRD (Fig. 5a). In the discharging process, the characteristic peaks gradually shifted to a low angle, indicating an expansion of the interstitial crystalline spacing. This fluctuation could be derived from the intercalation of H₂O molecules and Zn²⁺. After charging to 1.5 V, the reflection gradually shifted to its initial position, which suggests the reversibility of the electrode structural changes. New peaks appeared at 16.98° and 42.56° during the operating states, which revealed the formation of the Zn₃(OH)₂V₂O₇·2H₂O phase (JCPDS no. 50-0570). The detailed electrode can be summarized by the following formulas:⁴⁸

V⁵⁺ + 4H₂O → VO₂(OH)₂⁻ + 6H⁺ (6)

2VO₂(OH)₂⁻ + 3Zn²⁺ + 3H₂O → Zn₃(OH)₂V₂O₇·2H₂O + 4H⁺ (7)

Fig. 5 Structural characterization: (a) ex situ XRD at different charge and discharge states. XPS spectra of (b) Zn 2p, (c) V 2p and (d) O 1s in charging and discharging states. (e) Schematic of the Zn²⁺-storage mechanism in the NHVO-O_d-1 cathode.

Subsequently, ex situ XPS was employed to characterize the elemental valence changes of the electrodes. As shown in Fig. 5b, two distinct signal peaks could be observed at 1021.78 and 1044.98 eV, which corresponded to Zn 2p_3/2 and Zn 2p_1/2, respectively. The peak intensity of Zn 2p increased significantly as the discharge continued, which indicated that Zn²⁺ was successfully embedded in the lattice. Fig. 5c presents the XPS spectra of V 2p, with peaks assigned to V⁵⁺ (525.28/517.78 eV) and V⁴⁺ (524.18/516.58 eV), respectively. By integrating the area of the signal peaks, we obtained the ratios of the V⁴⁺ and V⁵⁺ contents. In the charging state, the ratio decreased from 1.04 to 0.61, which revealed the embedding of Zn²⁺ and occurrence of the redox reaction. Moreover, Fig. 5d demonstrates the XPS spectra of O 1s. As the pie charts show in the inset, the percentage oxygen content was obtained by integrating the area of each signal peak. When discharged to 0.3 V, there was a significant increase in the ratio of H₂O molecular (O–H). When charged to 1.5 V, the peak intensity of absorbed oxygen weakened, which indicated the involvement of both Zn²⁺ and H₂O.⁴⁹

The schematic in Fig. 5e depicts the conversion process of the batteries. To further understand the Zn²⁺-storage mechanism, the detailed cathode reaction processes involved are summarized as follows:

The 1st discharge process:

NH₄V₄O₁₀-O_d + xZn²⁺ + 2xe⁻ → Zn_xNH₄V₄O₁₀-O_d + Zn₃(OH)₂V₂O₇·2H₂O (8)

The subsequent cyclic processes:

Zn_xNH₄V₄O₁₀-O_d + Zn₃(OH)₂V₂O₇·2H₂O − (y + z)Zn²⁺ − 2(y + z)e⁻ ↔ Zn_x−yNH₄V₄O₁₀-O_d + Zn_3−z(OH)₂V₂O₇·2H₂O (9)

To further demonstrate the potential application of the prepared materials, we prepared flexible devices using NHVO-O_d-1 samples as the cathode. As presented in Fig. 6a, the cathode and anode were separated by PAM and subsequently sealed with an aluminum–plastic film. The mechanical property of the batteries was then investigated in different conditions. Fig. 6b shows the GCD curves at 1.0 A g⁻¹ under different bending angles. It could be observed that the curves maintained two platforms, which was basically similar to what was seen with button cell batteries. In the flat state, the discharged time of the battery reached about 1300 s. In the completely bent state, the discharge time significantly decreased. This could be attributed to the tight contact between the electrodes and the PAM becoming tighter under external folding pressure. When the device was restored to the freestanding state, the capacity manifested a reduction due to the loss of external pressure, which reduced the contact area. Fig. 6c displays the EIS spectra obtained for different bending degrees from 0° to 360°. The R_ct values gradually increased as the bending degree changed. When it returned to the initial folding condition, the corresponding value was close to that of the flat state, which indicates the high mechanical recoverability. At ambient temperature, the flexible devices delivered an initial capacity of 253.9 mA h g⁻¹ at 1.0 A g⁻¹. As shown in Fig. 6d, they maintained a retention ratio of 66.7% after 600 times cycling. Moreover, the batteries achieved a retention rate of 93.9% at 0 °C.

Fig. 6 (a) Flexible device structure schematic. (b) GCD curves in the 1.0 A g⁻¹. (c) EIS spectra. (d) Long-term cycles at 25 °C and 0 °C.

4. Conclusions

In summary, we synthetized oxygen defects (O_d)-containing NH₄V₄O₁₀ (NHVO-O_d) nanobelts through a hydrothermal route. During the synthesis procedure, the introduction of thiourea formed abundant O_d sites in the samples. The presence of O_d significantly promoted the electrical conductivity and improved the diffusion of Zn²⁺ and H₂O molecules. Benefiting from these merits, the assembled Zn//NHVO-O_d-1 coin batteries delivered an outstanding specific capacity and long cycle life. The soft pack device also showed favorable cycling stability under bending states. Moreover, the cathodes manifested excellent electrochemical performance at low temperature. It is hoped that this exploration in vanadate materials will prompt the further design of high-rate and long-cyclability energy-storage systems to operate under extreme operating conditions.

Author contributions

Yaotong Li, Chunru Zhao and Wei Wang: conceptualization, software, data curation and writing – original draft preparation, Xiang Wu and Yudai Huang: supervision, writing – reviewing and editing.

Data availability

The data and materials used to support the findings of this study are available from the corresponding author upon request.

Conflicts of interest

The authors declare no conflict of interest

Acknowledgements

This project is supported by National Natural Science Foundation of China (No. 52172218).

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Footnote

† These two authors contribute equally.

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