An in situ polymerization trial to prepare PEDOT decorated VO₂ hollow nanospheres for stable zinc ion storage at 0 °C

Abstract

Vanadium oxides possess a variety of crystal structures and have emerged as imperative cathode materials for zinc ion batteries in recent years. However, their further applications are largely restricted by their poor rate capability and cycling instability. In this work, we propose an in situ polymerization strategy to prepare novel VO₂ hollow nanospheres by PEDOT coating. The assembled Zn//VO₂@0.8PEDOT cells show a specific capacity of 455.8 mA h g⁻¹ at a current density of 0.2 A g⁻¹. They can still maintain a retention rate of 85.5% after 3000 cycles at 5.0 A g⁻¹. Additionally, the soft-packaged devices demonstrate remarkable mechanical stability under diverse folding conditions.

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Introduction

The emergence of large-scale energy storage systems and portable accessories has prompted researchers to concentrate on safe and cost-effective energy storage technologies.¹ In recent decades, lithium-ion batteries (LIBs) have become the preferred ones for the accumulator devices in electric vehicles owing to their high specific capacity and long cycle life.^2–4 However, their widespread application is hampered by the territorial distribution of lithium resources and high mining costs.⁵ Therefore, it is imperative to seek many alternatives to LIBs, such as rechargeable aqueous metal ion batteries (Na, Mg, Ca and Zn).^6–9 Among them, zinc ion batteries (ZIBs) demonstrate a certain development prospect due to their high theoretical capacity (820 mA h g⁻¹) and abundant resources.¹⁰ Various cathode materials have been reported for application in AZIBs, including manganese-based materials,^11,12 Prussian blue analogues^13–15 and vanadium-based products.^16,17

Among them, vanadium oxides have attracted significant attention owing to their plentiful valence states and open frameworks.¹⁸ The metastable phase VO₂(B) possesses a bilayer structure with large lattice spacing, based on edge-sharing VO₆ octahedra, which facilitates the rapid transfer of ions.¹⁹ The introduction of conductive polymers results in excellent electrochemical performances in electrode materials due to the expansion of interlayer space and high electrical conductivity. In the literature, Liu's group designed an ice bath route to synthesize polypyrrole (PPy) coated VO₂(A) hollow spheres.²⁰ The fabricated VO₂-5@PPy electrodes deliver a specific capacity of 440 mA h g⁻¹ at 0.1 A g⁻¹. Liu and co-workers²¹ prepared PVO@PEDOT nanosheets through an in situ intercalation and polymerization process. The assembled batteries deliver an energy density of 363.4 W h kg⁻¹ at a power density of 180 W kg⁻¹. Thus far, researchers have developed a variety of polymer modification strategies including polyaniline, polypyrrole and poly(3,4-ethylene dioxythiophene).^22–24

Herein, we utilize a simple hydrothermal route to synthesize VO₂ hollow spheres. The fabricated devices present a specific capacity of 290.5 mA h g⁻¹ at a current density of 0.2 A g⁻¹ and maintain 107.2 mA h g⁻¹ at 5.0 A g⁻¹ after 1000 cycles. The VO₂ hollow spheres coated with PEDOT consist of many nanosheets with interconnections, which largely increase the conductivity of the electrodes. The as-obtained VO₂@0.8PEDOT samples deliver a specific capacity of 455.8 mA h g⁻¹ at a rate of 0.2 A g⁻¹. Furthermore, they retain 275.9 mA h g⁻¹ at 5.0 A g⁻¹ even after 3000 cycles.

Experimental section

Preparation of VO₂ hollow spheres

The VO₂ samples were synthesized using a one-step hydrothermal process. Typically, 45 mL of deionized (DI) water was mixed with 0.7 g of NH₄VO₃ powder and stirred for 10 min at 80 °C. Next, 5 mL of 1 M HCl was added dropwise into the mixture for 5 min. Following that, 5 mL of N₂H₄·H₂O was added to the mentioned solution and stirred for 1 h. The above solution was subsequently transferred to an 80 mL Teflon-lined stainless-steel autoclave and maintained at 120 °C for 4 h. The synthesized products were repeatedly washed with alcohol and DI water several times, dried overnight at 60 °C and then annealed at 350 °C for 3 h under an Ar atmosphere. The collected black powder was named VO₂.

Preparation of VO₂@PEDOT products

0.3 g of the VO₂ sample was dispersed in an autoclave with 50 mL of DI water for 5 min. Then, 0.5/0.8/1.1 mL of EDOT monomer was added to the above solution respectively and stirred continuously at 25 °C for 24 h. During the stirring, the color of the mixture changes from gray to blackish green. The dark green products were centrifuged multiple times and dried overnight at 60 °C. The as-prepared samples were labeled as VO₂@0.5PEDOT, VO₂@0.8PEDOT and VO₂@1.1PEDOT, respectively.

Synthesis of PAM gel electrolyte

20 mL of 3 M Zn(CF₃SO₃)₂ electrolyte were mixed with 2 g of acrylamide powder and stirred until completely dissolved. Next, 10 mg of potassium persulfate and 2 mg of N,N′-methylenebis(acrylamide) were added to the above solution and stirred for 1 h. Finally, the slurry was injected into a mold (100 mm × 70 mm × 2 mm) and heated at 60 °C for 2 h.

Electrochemical characterization

Coin-type cells were assembled with a cathode, anode (zinc foil), separator (glass fiber membrane) and electrolyte (3 M Zn(CF₃SO₃)₂). The active material (VO₂@0.8PEDOT), acetylene black (Super p) and polyvinylidene fluoride (PVDF) were combined in a gravimetric ratio of 7 : 2 : 1. Subsequently, the mixture was placed into N-methyl-1-2-pyrrolidone (NMP) solution to form a slurry, which was evenly distributed on carbon paper. The cathode possesses an average loading mass of approximately 1.5 mg g⁻¹. The galvanostatic charge–discharge (GCD) curves were recorded and the galvanostatic intermittent titration technique (GITT) was performed using a Neware CT-4008T automatic battery tester. Finally, an electrochemical workstation (CHI660E) was used for cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) studies. All the electrochemical experiments were conducted within a cut-off voltage window range from 0.2 to 1.4 V.

Material characterization

We utilized X-ray diffraction (XRD, Rigaku Ultima IV) to characterize the crystal structure. X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha) was performed to ascertain the elemental composition of the samples. Then, their morphology and composition were observed by scanning electron microscopy (SEM, Gemini 300-71-31) and high-resolution transmission electron microscopy (HRTEM, FEI, Tecnai G2 F20). The surface and aperture structures were investigated using the Brunauer–Emmett–Teller (BJH, JW-TB220A) method. Finally, we measured the wettability of the electrode with a contact angle tester (JY-82C).

Results and discussion

First, we investigate the crystal structure and crystallinity of the samples by XRD, as shown in Fig. 1a. All diffraction peaks are indexed to the VO₂ phase (PDF#81-2392). Their lattice parameters are a = 12.093 Å, b = 3.702 Å and c = 6.433 Å. The typical peaks at 15.30°, 25.23°, 30.29°, 33.75°, 44.94°, 49.18°, 59.18° and 68.81° correspond to the (200), (110), (111), (−311), (−601), (020), (−404) and (−621) crystal planes. Furthermore, the VO₂@0.8PEDOT product presents strong peaks in the (200), (110) and (−311) planes, showing its excellent crystallinity.

Fig. 1 The crystal structure and chemical composition. (a) XRD patterns, (b) XPS full spectra, (c) V 2p, (d) O 1s, (e) C 1s, and (f) N₂ adsorption–desorption isotherms; the inset shows the pore size distributions of the VO₂ and VO₂@0.8PEDOT products.

Then, XPS is used to determine the elemental valence distribution of the samples. The V, O, C and N elements are shown in the full spectra (Fig. 1b). The V 2p spectra include 2p_1/2 and 2p_3/2 orbitals (Fig. 1c). The two peaks observed at 516.5 and 523.6 eV are deconvoluted into V⁴⁺, while the peaks at 517.7 and 525.1 eV are assigned to V⁵⁺.²⁵Fig. 1d presents the O 1s spectra at 532.52 and 531.26 eV, corresponding to V–O and O_d, respectively.²⁶ As seen in Fig. 1e, the C 1s spectra are split into three peaks, including the C–C bond at 284.8 eV, the C–O bond at 285.7 eV and C=O at 288.8 eV.²⁷ As seen in Fig. 1f, the VO₂@0.8PEDOT product demonstrates a larger specific surface area (82.654 m² g⁻¹) than the VO₂ material (53.337 m² g⁻¹). It suggests that the large specific surface area provides abundant active sites for the storage capacity of zinc ions.²⁸

We further observe the morphologies and structures of the samples by SEM and TEM. Fig. 2a and b show that VO₂ products present a hollow sphere shape. After the in situ polymerization of PEDOT, the nanoribbons are cross-linked with each other to form a porous network structure (Fig. 2c and d). It facilitates the penetration of electrolyte and the transfer of zinc ions.²⁹ The HRTEM images (Fig. 2e) clearly demonstrate a layer with amorphous characteristics on the surface of the samples. The lattice spacing of 0.206 nm corresponds to the (−601) crystal plane of the VO₂ phase. The elemental mappings indicate that four elements (V, O, C and S) are uniformly distributed along the surface of the materials, as shown in Fig. 2f.

Fig. 2 SEM images: (a and b) VO₂ and (c) VO₂@0.8PEDOT samples, (d) TEM image, (e) HRTEM images, and (f) EDS elemental mapping.

Subsequently, the obtained samples as cathodes are assembled into a series of coin-type cells to investigate the electrochemical kinetic behavior of the electrodes. The initial three CV curves of the VO₂@0.8PEDOT electrodes are illustrated in Fig. 3a, which recorded the voltage window of 0.2–1.4 at a scan rate of 0.1 mV s⁻¹. Fig. 3b shows the cycling performance at 0.2 A g⁻¹. The fabricated Zn//VO₂@0.8PEDOT cells deliver an initial specific capacity of 455.8 mA h g⁻¹. The specific capacity of the composites is significantly improved compared to that of Zn//VO₂ batteries (278.3 mA h g⁻¹). The platform of the GCD curves (Fig. 3c) is consistent with the redox peaks (1.03/0.92 and 0.72/0.47) of the CV curves.

Fig. 3 Electrochemical performance. (a) CV curves of VO₂@0.8PEDOT at 0.1 mV s⁻¹, (b) cyclability at 0.2 A g⁻¹, (c) GCD curves of VO₂@0.8PEDOT electrodes at 0.1 A g⁻¹, (d) long-term cycles at 5.0 A g⁻¹, (e and f) rate performance, (g) contact angle of the electrolyte (3 M Zn(CF₃SO₃)₂) on the VO₂ and VO₂@0.8PEDOT cathodes, (h) different temperatures, and (i) cycling performance at 0 °C.

Also, the rate capability of the batteries is a significant parameter in assessing their commercial applications. As depicted in Fig. 3d, the VO₂@0.8PEDOT coin-type cells show specific capacities of 479.6, 463.4, 443.7, 427.5, 404.6 and 349.4 mA h g⁻¹ at 0.1, 0.2, 0.5, 1.0, 2.0 and 5.0 A g⁻¹. A specific capacity of 470.3 mA h g⁻¹ (98.0% of the initial discharge capacity) is retained when the current density recovers to 0.1 A g⁻¹. From Fig. 3e, the VO₂@0.8PEDOT materials are further studied at a current density of 5.0 A g⁻¹ to evaluate the cycling stability of the batteries. The VO₂@0.8PEDOT cells deliver a specific capacity of 425.8 mA h g⁻¹ at 5.0 A g⁻¹ after 80 cycles activation. Moreover, they maintain a retention rate of 80.0% after 3000 cycles. The excellent mechanical stability is ascribed to the large contact area between the electrode and the electrolyte.³⁰ Nevertheless, VO₂ materials possess a retention rate of 47.5% after 1000 cycles. This is attributed to the repetitive embedded and detachment process of zinc ions, which ultimately leads to structural collapse.³¹Fig. 3f presents the corresponding charge/discharge curves from 0.1 to 5.0 A g⁻¹. It reveals that when the current density increases, the capacity decreases.

The interface behaviors of the VO₂ and VO₂@0.8PEDOT cathodes are first characterized through a water contact angle test. The contact angle (Fig. 3g) of the electrolyte (3 M Zn(CF₃SO₃)₂) on the VO₂ cathode reaches 25.1°, which is larger than that (21.0°) of the VO₂@0.8PEDOT cathode. Fig. 3h presents the variation of specific capacities with the change in the working temperature of the as-assembled batteries. The specific capacity of the device increases linearly with the temperature. When the temperature is 60 °C, the cell provides a specific capacity of 355.8 mA h g⁻¹ at a current density of 5.0 A g⁻¹. The specific capacitance retention of Zn//VO₂@0.8PEDOT is 87.5% at 0 °C after 100 cycles, as shown in Fig. 3i.

Also, we explore the electrochemical reaction kinetics of the battery and the storage behavior of zinc ions. Fig. 4a demonstrates the CV curves of the batteries at various scan rates from 0.1 to 0.5 mV s⁻¹. With the increase in the scan rate, the CV curve presents a similar shape. The oxidation peaks shift to a high voltage region, while the reduction peaks move to a low one. From Fig. 4b, according to the formula i = av^b, the calculated fitted b values are 1.14, 0.90, 0.92 and 0.91, which are all close to 1.³² The results indicate that the capacitive behavior is dominant during charging.³³ Then, the GITT curves are used to assess the diffusion capability of Zn²⁺ and the D_Zn value of the VO₂@0.8PEDOT electrode materials varies between 10⁻⁶ and 10⁻⁸ cm² s⁻¹ (Fig. 4c). This result shows that VO₂@0.8PEDOT enables the rapid transfer of charge to the electrode. EIS measurement is performed to further study the electrochemical behavior of the batteries. Also, the Nyquist plots are depicted in Fig. 4d, and the inset shows the schematic of the corresponding equivalent circuit. The VO₂@0.8PEDOT samples possess a lower charge transfer resistance (R_ct) than VO₂ products. This suggests that there is a fast ion diffusion process within the electrodes. Fig. 4e demonstrates the Ragone plot of VO₂@0.8PEDOT and other vanadium-based electrode materials. The Zn//VO₂@0.8PEDOT batteries show an energy density of 177.4 W h kg⁻¹ at a power density of 200 W kg⁻¹, which is better than that in previous reports.^34–39

Fig. 4 Electrochemical mechanism. (a) CV curves and (b) log(i) versus log(v) of the VO₂@0.8PEDOT electrode, (c) GITT curves, (d) Nyquist plots, and (e) Ragone plot of VO₂@0.8PEDOT.

To investigate the electrochemical reaction mechanism, ex situ XRD and XPS are utilized to study the storage mechanism and structural evolution of VO₂@0.8PEDOT samples. Fig. 5a presents the XRD patterns of the electrode in different charging and discharging stages. When the charge voltage decreases from 1.4 V to 0.2 V, the (−311) crystal plane moves from the low-angle region to the high-angle region. This is due to the expansion of the interlayer spacing of VO₂@0.8PEDOT materials with the insertion of water molecules and zinc ions during the reaction.⁴⁰ The diffraction peaks at 12.2° and 16.9° correspond to the (001) and (100) crystal planes of Zn₃V₂O₇(OH)₂·2H₂O (ZVO, JCPDS no. 87-0417) phases. This indicates that zinc ions are intercalated into the host successfully. The electrochemical reaction process can be expressed as follows:⁴¹

V⁵⁺ + 4H₂O → VO₂(OH)₂⁻ + 6H⁺ (1)

2VO₂(OH)₂⁻ + 3Zn²⁺ + 3H₂O → Zn₃V₂O₇(OH)₂·2H₂O + 4H⁺ (2)

Fig. 5 Structural characterization. (a and b) Ex situ XRD patterns, (c and d) XPS spectra, and (e and f) HRTEM images.

Fig. 5b shows the ex situ XRD patterns at a working temperature of 0 °C with different cycling times. Moreover, the by-products are obtained by the reaction of OH⁻ and Zn(CF₃SO₃)₂ and the reaction of Zn²⁺ and V⁵⁺, respectively. According to previous reports, OH⁻ is produced from the breaking of water molecules, which provides indirect evidence that protons are involved in the charge–discharge process.⁴²

Subsequently, we use ex situ XPS to explore the valence change and chemical constitution of VO₂@0.8PEDOT samples. The peak intensity of Zn 2p spectra in the completely charged and discharged states indicates that Zn²⁺ is successfully embedded/de-embedded in electrode lattices (Fig. 5c).⁴³Fig. 5d shows the ex situ V 2p spectra of the VO₂@0.8PEDOT electrode in various states. The peak area of different valence states changes with the change of charge and discharge states.⁴⁴ The result indicates the high reversibility of the PEDOT coated VO₂ electrode during the insertion/extraction of Zn²⁺.⁴⁵ Moreover, HRTEM is utilized to explore the reversibility of the structural evolution process, as seen in Fig. 5e and f. When the VO₂@0.8PEDOT electrode is discharged to 0.2 V, the (111) plane is extended to lattice fringes with a spacing of 0.304 nm due to the embedding of Zn²⁺. In the fully charged state, the (−601) plane recovers a spacing of 0.203 nm. This suggests that the VO₂@0.8PEDOT electrode possesses high structural stability.

Finally, a multitude of soft-packaged devices are fabricated to investigate the practical application of VO₂@0.8PEDOT products. Fig. 6a shows the structural schematic of a flexible battery. Fig. 6b shows the GCD curves of the flexible device at various folding angles at 1.0 A g⁻¹. The discharging time is extended with an increase in the bending angle owing to full contact between the electrode and hydrogel electrolyte. Fig. 6c shows that the R_ct values gradually decrease as the folding degree changes. The excellent mechanical stability is further demonstrated by the fact that it still restores to the initial state in the subsequent folding process. The soft-packaged device provides a retention ratio of 64.6% after 500 cycles at 1.0 A g⁻¹ (Fig. 6d). Finally, a blue LED is lit by connecting three VO₂@0.8PEDOT devices in series for 2 h, as shown in Fig. 6e.

Fig. 6 (a) Structure schematic of the flexible device, (b) GCD curves in the 1.0 A g⁻¹ state, (c) EIS, (d) long-term cycles at 1.0 A g⁻¹, and (e) digital photos of the blue LED.

Conclusion

In general, we have prepared zinc ion batteries with PEDOT coated VO₂ products as the cathode and 3 M Zn(CF₃SO₃)₂ as the electrolyte. The specific surface area and active sites are significantly increased by the addition of the PEDOT surfactant. Moreover, the Zn//VO₂@0.8PEDOT batteries demonstrate fast reaction kinetics and long-term cycle life. The electrical conductivity of PEDOT enhances the efficiency of electron and ion transport. Also, the as-fabricated cells show excellent specific capacity and rate performance at high current density. The full cells present outstanding mechanical durability at 0 °C. The soft-packaged cells achieve stable cycling, showing their promising potential for future energy storage applications.

Author contributions

Chunru Zhao, Yi Liu, Yefei Xu and Zhanyi Liu: conceptualization, software, data curation and writing – original draft. Xiang Wu, Mian Li and Qing Huang: supervision and writing – review and editing.

Data availability

The data supporting this article have been included in the submitted article.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

The work is supported by the National Natural Science Foundation of China (no. 52472227).

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