Flexibility-Engineered Nb₂O₅ Carbon Nanofiber Film Anodes: Concentration-Dependent Optimization for Mechanically Robust and Stable Sodium Storage

Abstract

This study systematically investigates the correlation between the concentration of niobium oxide (Nb₂O₅) in carbon nanofibers (CNFs) and the microstructural characteristics and electrochemical performance of the composites. Through rational optimization of Nb₂O₅-to-CNFs mass ratios, we demonstrate that 50% Nb₂O₅ CNFs composite achieves an optimal balance between enhanced rate capability and structural stability. The composites transition from brittle to flexible with increasing Nb₂O₅ content, achieving unprecedented bending durability at 50% loading. This mechanical–electrochemical synergy positions Nb₂O₅ CNFs as viable candidates for flexible energy storage devices. Comprehensive characterization reveals that appropriate Nb₂O₅ incorporation significantly improves specific capacity (181 mA h g⁻¹ at 0.1 A g⁻¹) and rate performance compared to pristine CNFs. However, excessive Nb₂O₅ doping (>50%) induces detrimental hydrolysis reactions during synthesis, compromising both material processability and electrochemical reversibility. This work contributes to the development of flexible self-supporting frameworks that are superior to hard carbon for constructing high-performance flexible sodium-ion batteries.

1. Introduction

Sodium-ion batteries have emerged as promising alternatives to lithium-ion systems due to sodium’s natural abundance and cost-effectiveness [1,2,3]. Nevertheless, the development of suitable anode materials remains challenging, particularly in overcoming the sluggish Na+ diffusion kinetics and structural instability during cycling [4,5,6,7,8,9]. Niobium pentoxide (Nb₂O₅) has attracted growing attention owing to its intercalation pseudocapacitance behavior and suitable working potential (~1.0 V vs. Na/Na⁺). However, its practical implementation is hindered by intrinsic low electronic conductivity and particle aggregation issues [10,11,12,13,14,15,16].

Recent advances propose combining Nb₂O₅ with conductive carbon matrices to address these limitations [17,18,19,20,21,22,23,24,25]. While our previous studies have systematically investigated the electrochemical properties of Nb₂O₅ [26,27,28], its role in modulating electrode flexibility remains overlooked. The inherent brittleness of carbon-based anodes severely limits their application in flexible electronics, necessitating material designs that harmonize ionic conductivity, electron transport, and mechanical resilience [29,30,31]. Particularly, electrospun carbon nanofibers (CNFs) offer three-dimensional conductive networks and mechanical flexibility [32,33,34]. Despite these merits, the critical relationship between Nb₂O₅ concentration and resultant electrochemical performance remains insufficiently explored.

Different from previous research work, this study bridges the gap between flexible engineering and electrochemical optimization between Nb₂O₅ and CNFs, providing actionable guidelines for designing flexible, binder-free electrodes. Herein, we present the systematic investigation of Nb₂O₅ CNFs composites with controlled mass ratios (0%–60%). Through integrated material characterization and electrochemical analysis, we establish that 50% Nb₂O₅ incorporation achieves an optimal sodium storage performance while maintaining structural integrity. This work elucidates the concentration-dependent behavior of metal oxide/carbon hybrids, providing essential guidelines for advanced flexible anode design.

2. Materials and Methods

2.1. Materials

Niobium pentaethoxide (Nb(C₂H₅O)₅, 99.95% trace metals basis), N,N-dimethylformamide (DMF, 99%), and polyacrylonitrile (PAN, average Mw = 150,000) were obtained commercially and utilized as received without additional purification.

2.2. Methods

The precursor solution was prepared by dissolving niobium pentaethoxide (Nb(C₂H₅O)₅) and 0.75 g polyacrylonitrile (PAN) in 10 mL dimethylformamide (DMF), followed by continuous magnetic stirring for 12 h at room temperature. Subsequently, the homogeneous solution was loaded into a plastic syringe equipped with a 24-gauge stainless-steel needle for electrospinning. The electrospinning process was conducted under optimized parameters: an applied voltage of 15 kV, a feeding rate of 1 mL h⁻¹, and a working distance of 15 cm between the needle tip and the rotating collector.

The as-spun nanofiber mats were thermally stabilized in a muffle furnace under ambient air via a two-step protocol: pre-oxidation at 250 °C for 3 h with a heating rate of 2 °C min⁻¹, followed by carbonization in a tubular furnace under an argon flow. The carbonization process involved ramping to 700 °C at 2 °C min⁻¹ and maintaining this temperature for 5 h to yield Nb₂O₅ embedded carbon nanofibers (Nb₂O₅ CNFs).

To systematically investigate the concentration-dependent effects, six samples were synthesized by varying the Nb(C₂H₅O)₅ mass added to the precursor solution: named as pristine CNFs, 10% Nb₂O₅ CNFs, 20% Nb₂O₅ CNFs, 30% Nb₂O₅ CNFs, 40% Nb₂O₅ CNFs, 50% Nb₂O₅ CNFs, and 60% Nb₂O₅ CNFs, respectively. The mass percentages were calculated based on the Nb₂O₅-to-PAN weight ratio in the precursor.

2.3. Characterizations

The crystal structures of all samples were examined using X-ray diffractometry (XRD, DX-2700BH, Haoyuan, Dandong, China). The microscopic morphology of the anode were investigated through field emission scanning electron microscopy (SEM, GeminiSEM 300, ZEISS, Oberkochen, Germany).

2.4. Electrochemical Test0073

All other samples served as working electrodes directly, without the use of current collectors, binders, or conductive additives. Self-supporting samples were directly utilized as CR-2032 half-cell anodes without employing current collectors, binders, or conductive additives. The sodium metal disk functioned as both the working and reference electrodes, with a glass fiber membrane used acting as the separator. The electrolyte comprised 1 mol L⁻¹ sodium perchlorate dissolved in 100% propylene carbonate (PC) and 5% fluorinated ethylene carbonate (FEC). Electrochemical charge–discharge tests were performed using the BTS-4000 battery testing system (Neware, Shenzhen, China).

3. Results and Discussion

The fabrication process of Nb₂O₅ CNFs with varying concentrations is schematically illustrated in Figure 1. Utilizing polyacrylonitrile (PAN) and niobium ethoxide as precursors, the composite nanofibers were synthesized through electrospinning followed by thermal stabilization and carbonization. Mechanical flexibility tests, as demonstrated in Figure 2, revealed a striking concentration-dependent behavior. Pristine CNFs exhibited extreme brittleness, fracturing into fragments upon manual folding. In contrast, the 10% Nb₂O₅ CNFs showed partial flexibility improvement, sustaining single-fold deformation despite developing permanent crease cracks. Notably, the 30% Nb₂O₅ CNFs achieved reversible shape recovery after three-fold creasing, while the 50% composite displayed exceptional self-recovery capability, fully restoring its original morphology even after aggressive crumpling.

The crystalline structures of Nb₂O₅ CNFs composites were first analyzed by XRD (Figure 3a). All diffraction patterns corresponded to orthorhombic Nb₂O₅ (JCPDS 30-0873), confirming successful phase formation. Notably, the diffraction peak intensities proportionally increased with niobium ethoxide content, verifying controllable Nb₂O₅ crystallization within the carbon matrix. The reasons may be attributed to the uniform distribution of Nb₂O₅ nanoparticles acting as the buffer substances to efficiently weaken and disperse the bending stresses, and simultaneously absorb the stress energy during the folding process. In addition, Figure 3b shows the crystal structure of Nb₂O₅. As we can see, the Nb-ions are surrounded by six O-ions, the (001) planes of Nb₂O₅ have a large interplanar spacing of 0.39 nm, which could provide stable and effective tunnels for Na-ion storage and transport.

To probe the microstructural features, SEM observations were conducted, as shown in Figure 4. As we can see, all samples exhibited continuous network structures formed by interconnected fibers with diameters of approximately 200 nm. Remarkably, no discernible morphological differences were observed among the various compositions with the different Nb₂O₅ additive amount, suggesting that the electrospinning process effectively accommodated different precursor ratios.

Electrochemical evaluation demonstrated significant performance dependence on Nb₂O₅ content (Figure 5a). At 0.2 A g⁻¹, the 50% Nb₂O₅ CNFs delivered 153 mA h g⁻¹, outperforming pristine CNFs (124 mA h g⁻¹) by 23.4%. Rate capability tests demonstrated superior kinetics, with the 50% Nb₂O₅-CNFs retaining 50 mAh g⁻¹ at 10 A g⁻¹, surpassing the capacities of 40% (44 mA h g⁻¹), 60% (27 mA h g⁻¹), and pristine CNFs (38 mA h g⁻¹) counterparts. This enhancement originates from the pseudocapacitive contribution of Nb₂O₅ and improved ion accessibility within the hybrid structure.

Figure 5b presents the galvanostatic charge–discharge (GCD) profiles of the 50% Nb₂O₅ CNFs anode during the first five cycles at 0.1 A g⁻¹. For the rate performance, all anodes demonstrated a higher specific capacity and lower coulombic efficiency during the first charge–discharge cycle compared to the subsequent cycles. This can be attributed to the formation of the solid electrolyte interphase (SEI) layer and the irreversible insertion of sodium ions.

To comprehensively assess the Na⁺ storage capabilities arising from intercomponent collaboration, extended cycling stability tests were conducted under elevated current conditions (1 A g⁻¹), as depicted in Figure 5c. The composite demonstrated exceptional stability across 1800 cycles, maintaining 96% capacity preservation rate with Coulombic efficiency values remaining within 99.8 ± 0.3%. Quantitative analysis of galvanostatic profiles (Figure 5d) revealed remarkable consistency in voltage plateaus and polarization evolution throughout cycling, corroborating outstanding interfacial stability and reaction reversibility.

Figure 5e displays representative cyclic voltammetry profiles for the initial five cycles of fabricated specimens recorded at a scanning rate of 0.5 mV s⁻¹ within an operational voltage range of 0.01–3.0 V (vs. Na⁺/Na). Regarding the 50% Nb₂O₅ CNFs composite anode, apart from the irreversible processes observed during the first cycle, the subsequent cycles (second to fifth) display pronounced overlapping characteristics, demonstrating the excellent electrochemical reversibility and cycling durability of sodium ion storage mechanisms.

The optimized 50% Nb₂O₅ CNFs achieve a critical balance: Nb₂O₅ nanoparticles can achieve pseudocapacitive Na-ion storage (50 mA h g⁻¹ at a very large current density of 10 A g⁻¹), while the carbon matrix can provide sample conductivity, accompanied by excellent flexibility. The reasons may be attributed to the uniform distribution of Nb₂O₅ nanoparticles, which act as buffering agents to effectively mitigate and distribute the bending stresses, and simultaneously absorb the stress energy during the folding process. For the mechanism of Nb₂O₅ pseudocapacitive sodium storage, galvanostatic intermittent titration technique (GITT), and first-principles calculations based on density functional theory (DFT), please refer to our previous studies [26,27,28].

4. Conclusions

This work establishes a critical composition–property–performance relationship in Nb₂O₅ carbon nanofibers (CNFs) composites for advanced sodium-ion battery anodes. The 50% Nb₂O₅ CNFs composite emerges as the optimal formulation, delivering a specific capacity of 153 mA h g⁻¹ at 0.2 A g⁻¹ alongside exceptional bending durability. This performance stems from the synergistic interplay between Nb₂O₅’s pseudocapacitive Na⁺ storage and the CNFs conductive 3D network, which collectively enhance ion kinetics and mechanical resilience. Furthermore, the self-supporting electrode architecture eliminates conventional binders and current collectors, achieving 96% capacity retention over 1800 cycles at 1 A g⁻¹, thus demonstrating the practical viability of flexible energy storage devices. We believe that 50% Nb₂O₅ CNFs prepared in this work can replace pure CNFs as a superior self-supporting framework, given its better flexibility and sodium storage performances.