3D Interconnected V6O13 Nanosheets on Carbonized Textile: A High‑Performance Flexible Cathode for Lithium‑Ion Batteries

Abstract

Three‑dimensional, free‑standing nanostructures have emerged as highly promising electrodes for energy storage, delivering superior electrochemical performance and enabling wearable power solutions. In this study, we grew interconnected V₆O₁₃ nanosheets directly onto a flexible carbonized textile (c‑textile) using a seed‑assisted hydrothermal method, forming a 3‑D free‑standing electrode for lithium‑ion batteries (LIBs). The pristine electrode delivers a specific capacity of 170 mA h g⁻¹ at 300 mA g⁻¹. A subsequent coating with carbon nanotubes (CNTs) further enhances the capacity by 12–40 % across various current rates, while maintaining a reversible capacity of 130 mA h g⁻¹ (74 % of the initial value) after 300 cycles at 300 mA g⁻¹. These metrics surpass most mixed‑valence vanadium oxides, owing to the synergistic 3‑D nanostructure that promotes Li⁺ diffusion and the hierarchical conductive network formed by CNTs and the carbon fiber scaffold.

Background

Vanadium oxides—V₆O₁₃, V₃O₇, V₂O₅—are attractive cathode materials for LIBs due to their low cost, high theoretical capacity, and the abundant supply of vanadium [1–6]. V₆O₁₃ is especially compelling, with a theoretical capacity of 417 mA h g⁻¹ and an energy density of 890 Wh kg⁻¹ when fully lithiated to Li₈V₆O₁₃ [2, 8]. However, its practical performance is hampered by a drop in electronic conductivity upon lithiation and modest Li⁺ diffusion coefficients (10⁻⁸–10⁻⁹ cm² s⁻¹) [7, 9]. Three‑dimensional nanostructuring mitigates these issues by enhancing ion and electron transport while preventing particle aggregation [15–20]. Prior work has demonstrated 3‑D V₆O₁₃ nanostructures on rigid substrates, yet direct growth of mixed‑valence vanadium oxide on flexible supports remains challenging, and flexible cathodes for wearable devices have not been reported.

In this work, we present a straightforward hydrothermal route that successfully grows interconnected V₆O₁₃ nanosheets on carbonized bamboo textile, producing a 3‑D free‑standing electrode. The resulting cathode delivers 161 and 105 mA h g⁻¹ at 300 and 1200 mA g⁻¹, respectively. CNT coating further raises the capacities to 170 and 140 mA h g⁻¹ and improves cycle life, retaining 74 % of the initial capacity after 400 cycles at 300 mA g⁻¹. These enhancements are attributed to the synergistic effect of the 3‑D nanostructure and the hierarchical conductive network.

Methods

Synthesis of c‑textile

Commercial bamboo cloth was soaked in a 2.5 g NaF/60 mL H₂O solution for 1 h, then dried at 120 °C for 5 h. The dried textile was carbonized at 800 °C under nitrogen for 30 min, yielding the flexible c‑textile.

Growth of 3D V₆O₁₃ nanostructure on c‑textile

The c‑textile was briefly oxidized in 80 wt % nitric acid (30 min) to introduce oxygen‑bearing functional groups. A 1 mg V₂O₅ suspension in 5 mL deionized water was sonicated for 15 min to create a seed solution. The oxidized textile was immersed in this suspension for 2 h, dried, and heated at 300 °C for 10 min to nucleate vanadium oxide seeds. Subsequently, 16 mg V₂O₅ was added to 224 µL of 30 wt % H₂O₂, stirred 10 min, and diluted with 40 mL water; the mixture was stirred 30 min before being transferred to a 25 mL stainless‑steel autoclave. The oxidized c‑textile was immersed, and the autoclave was maintained at 180 °C for 48 h. After washing with water and alcohol and drying at 60 °C for 8 h, a flexible 3‑D V₆O₁₃ electrode was obtained. CNTs were then coated by repeatedly dipping the electrode in a 0.5 mg mL⁻¹ NMP suspension of multi‑walled CNTs and drying, producing the V₆O₁₃/CNT composite.

Characterization of Materials

Morphology was examined by scanning electron microscopy (SEM, Philips XL30 FEG) and transmission electron microscopy (TEM, JEOL JEM‑2010). X‑ray photoelectron spectroscopy (XPS, K‑Alpha) was performed with a monochromatic Al Kα source.

Battery Fabrication and Electrochemical Measurements

CR2016‑type coin cells were assembled in an argon glove box, using the V₆O₁₃ electrode (≈1 mg cm⁻²) as the cathode and lithium foil as the anode. The electrolyte was 1 M LiPF₆ in a 1:1:1 volume mixture of EC/DEC/DMC, and a polypropylene separator was employed. Cells were cycled between 1.5 and 4.0 V (vs. Li/Li⁺) at 25 °C on a LAND battery test system. Electrochemical impedance spectroscopy (EIS) was performed using an Autolab PGSTAT302N workstation over 10 mHz–10 kHz.

Results and Discussion

The growth process of the 3‑D V₆O₁₃ nanosheets on c‑textile is illustrated in Additional file 1: Figure S1. Initially, the bamboo cloth (Fig. 1a) was carbonized to produce c‑textile (Fig. 1b). SEM images (Fig. 2a) show that the c‑textile consists of woven bundles of carbon fibers (~5 µm diameter), exhibiting excellent flexibility and a sheet resistivity of 5 Ω sq⁻¹. After hydrothermal growth, the textile becomes coated with a thin yellow‑green film (Fig. 1d) while remaining flexible. The resistivity increases to 50 Ω sq⁻¹, yet the structure remains intact. SEM (Fig. 2b,c) reveals interconnected nanosheets several microns long and a few hundred nanometers wide, forming the 3‑D architecture. High‑resolution TEM (Fig. 2f) displays lattice fringes with a spacing of 3.5 Å, matching the (110) plane of orthorhombic V₆O₁₃ (PDF No. 71‑2235). XPS confirms the mixed valence of vanadium, with V⁴⁺ (516.0 eV) and V⁵⁺ (517.3 eV) peaks, consistent with literature values [25–27]. These results confirm successful growth of 3‑D V₆O₁₃ on c‑textile.

3D Interconnected V6O13 Nanosheets on Carbonized Textile: A High‑Performance Flexible Cathode for Lithium‑Ion Batteries

Electrochemical performance was assessed in half‑cell configuration. CV curves (Fig. 4a) show distinct redox peaks at 2.8/2.5 V, with additional broad peaks at ~3.2 V and 2.3 V (anodic) and ~1.8 V (cathodic), indicating multi‑step phase transitions consistent with prior reports [2, 28]. Galvanostatic discharge/charge at 30 mA g⁻¹ (Fig. 4b) exhibits sloped plateaus at 2.3 and 2.8 V. Specific capacities at 30, 150, 300, 600, and 1200 mA g⁻¹ are 253, 176, 161, 133, and 105 mA h g⁻¹, respectively, highlighting the advantage of the open 3‑D architecture for Li⁺ transport. SEM of the cycled electrode (Additional file 1: Figure S2) confirms that the nanosheet morphology is preserved, underscoring structural integrity.

To further enhance conductivity, the electrode was coated with CNTs. SEM (Fig. 5a,b) shows CNTs uniformly deposited on the nanosheets, forming inter‑sheet bridges (Fig. 5c). The sheet resistivity drops from 50 to 20 Ω sq⁻¹. CV peaks remain at the same potentials but with increased current, indicating faster kinetics. Rate performance (Fig. 4c) improves: capacities of 261, 185, 170, 153, and 140 mA h g⁻¹ at 30, 150, 300, 600, and 1200 mA g⁻¹ represent a 12–40 % boost over the uncoated electrode. Li⁺ diffusion coefficients calculated from CV are 4.79 × 10⁻⁸ and 2.01 × 10⁻⁸ cm² s⁻¹ for the anodic and cathodic processes, respectively, higher than those of the bare electrode (2.42 × 10⁻⁸ and 1.70 × 10⁻⁸ cm² s⁻¹).

EIS Nyquist plots (Fig. 6a) show a reduced charge‑transfer resistance (R_ct) of 37.24 Ω for the CNT‑coated electrode versus 55.58 Ω for the bare one, attributable to the CNT‑mediated conductive network. The equivalent circuit (inset) includes a constant phase element and Warburg impedance to account for Li⁺ diffusion.

Cycling stability (Fig. 4d) is markedly improved: the CNT‑coated electrode retains 74 % of its initial capacity after 300 cycles at 300 mA g⁻¹, compared to 42 % for the bare electrode. The enhanced durability is ascribed to mechanical reinforcement by CNTs, continuous attachment of nanosheets to the conductive scaffold, and suppression of side reactions. Future improvements may involve reducing c‑textile resistance, doping V₆O₁₃ with sulfur, or applying conductive polymer coatings.

Conclusions

We have demonstrated a facile seed‑assisted hydrothermal route to grow 3‑D, free‑standing V₆O₁₃ nanosheets directly on a flexible carbonized textile, producing a high‑performance, flexible cathode for LIBs. The electrode delivers robust capacity and excellent cycle life, further enhanced by CNT coating, which establishes a hierarchical conductive network and improves both electronic and ionic transport. This strategy—combining 3‑D nanostructured building blocks with a hierarchical conductive scaffold—can be extended to other electrode chemistries to achieve superior energy‑storage performance.