TEOS-Based Fiber Fabrication via Electrospinning: Influence of Process Parameters and NMC Doping on Functional Properties

Abstract

The main aim of this study is to produce TEOS-based fibers using the electrospinning method with solutions without carrier polymers, unlike most TEOS-based fibers that are produced with polymer additives. This study provides fundamental insights into the production and characterization of TEOS-based fibers and offers a general overview of their potential applications. We investigate the production and overlaying of their morphological, chemical, thermal, and electrochemical properties. The effects of electrospinning parameters such as voltage, flow rate, and solution viscosity on fiber morphology were examined, revealing a strong dependence of fiber diameter and structural uniformity on these parameters. Furthermore, TEOS-based fibers containing nickel–manganese–cobalt oxide (NMC) were fabricated, and their electrochemical behavior was investigated. The analyses indicate that the addition of NMC enhances the electrochemical properties of the TEOS fibers; however, the system still requires further improvement to be effective in energy-storage applications. To investigate how the flow properties of the solution affect fiber generation during electrospinning, viscosity measurements were conducted on the TEOS-based solution. Differential thermal analysis (DTA) was applied to assess the thermal behavior and stability of the fibers at elevated temperatures. The produced fibers were analyzed using various characterization techniques. As a result, thin fibers were successfully produced.

1. Introduction

The last few years have witnessed significant advancements in nanotechnology and materials science. In particular, fibrous structures stand out due to their superior mechanical, thermal, and chemical properties. Nanofibers play a crucial role in various industrial applications due to their high surface area and small diameters. They possess significant potential in various fields such as energy storage, biomedical engineering, environmental filtration, sensors, and protective coatings. Additionally, fibers with larger diameters are commonly utilized across various industries. Thus, creating fibers at both the micro- and nanoscale holds significant value in materials science and engineering [1]. The production of fibers varies significantly depending on the material type, production techniques, and end-user applications. In this context, silica-based compounds, particularly TEOS (tetraethyl orthosilicate), are commonly used for fiber production. TEOS, an inorganic silica-based compound, can be transformed into fibers using sol–gel processes. Silica possesses high mechanical strength, chemical stability, and resistance to elevated temperatures. Therefore, TEOS and its derivatives are widely used in fiber production, and these fibers have found extensive applications across various fields [2,3,4].

There are several methods for fiber production, and one of the most notable is the electrospinning technique. Electrospinning employs a high electric field to draw fine fibers from a solution. This method enables the production of structures ranging from nanofibers to microfibers and provides control over fiber diameter, morphology, and other physical properties through parameters such as solution viscosity, voltage, and flow rate. Furthermore, based on the selected material and the specific composition of the solution, this method provides the versatility to fabricate both homogeneous and heterogeneous fiber [5,6,7].

Electrospun nanofibers typically have diameters ranging from 1 nm to 1 µm and can be produced in lengths of several meters. As a result, nanofibers have a much higher length-to-diameter ratio compared with microfibers or bulk counterparts, providing superior mechanical strength, flexibility, surface area-to-volume ratio, porosity, gas permeability, and structural stability. While their diameters usually range between 100 nm and 800 nm, mass production of fibers with diameters as small as 36 nm and small-scale production of fibers with diameters of 2 nm are possible [8,9].

Silica-based solutions such as TEOS, prepared through the sol–gel method, are ideal for fiber production using electrospinning. The sol–gel technique is essential in fabricating silica-based fibers, as it enables the development of uniform and well-regulated structures at relatively low processing temperatures. Fibers made using this method demonstrate outstanding properties, including high thermal resistance, chemical inertness, and biocompatibility. Furthermore, silica-based fibers prepared via the sol–gel method offer advantages such as electrical conductivity, mechanical strength, and high surface area, making them highly promising for energy storage and electronic applications, among many others [10,11,12].

Electrospinning is a fiber fabrication technique that applies an electric field to produce continuous, solid fibers with diameters ranging from the microscale to the nanoscale [13]. Electrospinning was first performed in 1747 by Abbé Nollet and patented in the early 1900s by John Cooley and William Morton [14]. In 1914, John Zeleny studied the behavior of liquid droplets behave at the tips of metal capillaries, marking the beginning of mathematical modeling for liquid behavior under electrostatic forces [15]. Later, in 1934, Anton Formhals patented the earliest method for producing nanofibrous materials from polymer solutions [16,17]. A standard electrospinning system generally comprises a high-voltage power supply, a syringe needle, and a collecting screen. Figure 1 illustrates a typical electrospinning setup [18].

The basic principle of electrospinning involves managing the process by which low-molecular-weight, conductive liquid droplets, formed under vacuum, are subjected to two types of forces. These forces include a repulsive force that acts against the spherical shape of the droplets, and surface tension. This process applies a high-voltage electric current to a polymer solution [19]. Upon application of a high voltage (up to 30 kV) between the spinneret and the collector, electrostatic forces act on the charged solution at the needle tip, deforming it into a conical shape commonly referred to as the Taylor cone [20]. When the applied voltage surpasses the threshold needed to overcome surface tension, a fine jet emerges from the apex of the Taylor cone [21]. This jet traverses the set distance between the needle tip and the collector, which is typically a metallic foil. Between the needle and the collector, nanofibers experience two primary stresses: radial tension resulting from the repulsion between positively charged polymer chains and longitudinal tension due to the strong attraction of the collector. The emergence of the Taylor cone at the needle tip results from the first tension effect, while the resulting jet solidifies into a randomly aligned web of nanofibers on the grounded collector surface [22,23,24]. The electrospinning process is affected by several factors, which can be broadly grouped into solution parameters, process parameters, and environmental parameters. Solution-related parameters refer to properties such as viscosity, conductivity, molecular weight, and surface tension, while processing parameters include electric field strength, distance between the needle and the collector, and the solution delivery rate. Each of these factors significantly influences the resulting fiber morphology, and careful adjustment allows the production of nanofibers with controlled structures and diameters [25,26,27,28]. Furthermore, ambient conditions, including humidity and temperature, significantly influence both the structural features and size of nanofibers produced via electrospinning [29,30,31,32,33].

While this study primarily focuses on the production and characterization of TEOS-based fibers, it also provides a general overview of their possible applications. This work aims to contribute to materials engineering and industrial production processes by exploring the potential applications of both nanofibers and larger-diameter fibers. Specifically, their potential use in battery systems and/or in many areas represents a unique aspect of this study.

2. Experimental Procedure

Materials and Solution Preparation

Tetraethyl orthosilicate (TEOS, Sigma-Aldrich), ethanol (EtOH), hydrochloric acid (HCl), nickel–manganese–cobalt oxide nanopowders (NMC), and deionized water were used. In this experiment, the sol–gel and electrospinning methods were used for the production of silica-based fibers. Tetraethyl orthosilicate (TEOS) was chosen as the silica precursor. A solution was prepared by mixing 50.00 g of TEOS with 5.44 g of ethanol, and the pH of this solution was adjusted using a titration device (Titronic basic). To control the viscosity and pH of the prepared mixture, the RB solution was slowly added to the TEOS and ethanol solution via the titration device. A mixture of deionized water (544.00 g), methanol-denatured ethanol (348.17 g), and 36% hydrochloric acid (5.12 g) was used to prepare the RB solution. The addition of the RB solution was precisely controlled using the titration device, with a controlled addition of 0.01 mL every 6 s until the total volume added reached 15.6 mL. A low concentration of HCl was added to initiate hydrolysis of TEOS and facilitate the formation of silica structures within the mixture. This step plays a key role in promoting mixture uniformity and obtaining the targeted morphology in the resulting fiber. The hydrolysis and condensation of TEOS lead to the formation of silica structures within the mixture, which can affect the surface properties and durability of the final fibers. Subsequently, the mixture underwent heat treatment at 80 °C for three hours. To prepare the NMC-containing fibers, 10% (w/w) of NMC was added to 10 g of thermally treated TEOS-based solution.

Table 1. Materials used in the experiment.

Material	Chemical Formula	Amount Used	Description
Tetraethyl orthosilicate (TEOS)	Si(OC2H5)4	50.00 g	Precursor for silica-based nanofiber production
Ethanol	C2H5OH	5.44 g	Solvent
RB solution (Deionized water)	H2O	544.00 g	Reactant used in the hydrolysis reaction
RB solution (Denatured ethanol)	C2H5OH + CH3OH	348.17 g	Solvent (ethanol denatured by methanol)
RB solution (36% HCl)	HCl 36%	5.12 g	Used to adjust the pH and catalyze the hydrolysis of TEOS
Volume Added (RB Solution) Nickel–Manganese–Cobalt (NMC) nanoparticles	C2H5OH + CH3OH+ HCl 36% LiNi0.6Mn0.2Co0.2O2 (NMC622)	15.60 mL 1 g	Added in a controlled manner Added to improve electrochemical performance

lists the experimental materials, their respective quantities, and their roles in the process.

The EF050 electrospinning starter kit (Figure 2) was used for electrospinning.

The solution prepared for electrospinning was drawn into a 10 mL syringe, which was then placed in the injection pump (Figure 2, left). In front of the metal collector (Figure 2, right), an aluminium plate was placed, onto which a composite grid, primarily made of plastic and reinforced with glass fiber, was attached using rubber bands. The micro/nanofibers were then collected on this grid. A blunt-ended 17-gauge stainless steel needle was attached to the center of the device. To establish the electric field necessary for electrospinning, the needle was connected to one terminal of the high-voltage power supply, while the metallic collector was grounded. The solution was pushed by a syringe through the tube into the needle, where the charge was transferred to the polymer solution. The polymer droplet at the tip of the needle was then transformed into a Taylor cone by electric forces, from which micro/nanofibers were then formed during their flight from the tip of the needle to a metal collector.

Table 2. Parameters used for electrospinning in each sample.

Sample No	Needle-to-Collector Distance (cm)	Voltage (kV)	Solution Flow Rate (mL/h)	Solution Volume Per Sample (mL)	Needle Inner Diameter (mm)
TB1	15 cm	18 kV	2 mL/h	3 mL	1.067 mm
TB2	15 cm	26.5 kV	1.5 mL/h	3 mL	1.067 mm
TBN1	15 cm	25 kV	2.5 mL/h	5 mL	1.067 mm
TBN2	15 cm	24.5 kV	1.25 mL/h	5 mL	1.067 mm

presents the electrospinning parameters used for each sample.

Various characterization techniques were employed to analyze the physical and chemical properties of the produced fibers. The morphology and diameter of the produced silica fibers were analyzed using Scanning Electron Microscopy (SEM, TESCAN VEGA 3LMU, Czech Republic). The surface of the mats and distribution characteristics of the fibers were evaluated using Scanning Laser Confocal Microscopy (Olympus LEXT OLS5000, Japan). The viscosity of the solution was measured with a viscosimeter (Thermo Scientific HAAKE RV1, Germany) and analyzed in terms of its suitability for fiber formation and electrospinnability. Energy-Dispersive Spectroscopy (EDS, OXFORD Instruments INCA 350, Oxford, UK) was used to determine the elemental distribution. The chemical composition was identified by Fourier Transform Infrared Spectroscopy (FTIR, Nicolet iS50, Thermo Fisher Scientific, USA). The crystal structure of the silica fibers was analyzed using X-ray Diffraction (XRD, X’Pert³ Powder θ-θ powder diffractometer, PANalytical, Holland). The thermal behavior and high-temperature stability of the fibers were examined through Differential Thermal Analysis (DTA, Linseis STA PT 1600/1750 °C HiRes, Germany). Measurements were conducted using a Linseis STA PT Series device in TG-DTA mode, with approximately 30 ± 0.05 mg of sample, under an air atmosphere, at a heating rate of 10 °C/min from room temperature to 1000 °C.

In addition, Electrochemical Impedance Spectroscopy (EIS, VIONIC, Metrohm, Switzerland) was performed to evaluate the electrochemical performance of both undoped TEOS and NMC-doped TEOS fibers.

3. Results and Discussion

Fibers were produced under varying electrospinning parameters to investigate the effects of voltage and solution concentration on fiber morphology and structure. Four fiber samples were selected for characterization. As illustrated in Figure 3, two samples include NMC, which is significant for battery-related studies, while the remaining two consisted solely of silica. These samples were compared to evaluate how production conditions and compositional differences influenced their morphological, chemical, and thermal properties.

The first undoped TEOS fiber sample (TB1), shown in Figure 3a, was produced using a low voltage (18 kV) and a moderate extrusion rate (2 mL/h). The low voltage reduced the stability of the electrospinning jet, resulting in irregular fiber formation. Additionally, compared to the second undoped TEOS fiber sample (TB2) in Figure 3b, the higher extrusion rate led to the formation of thicker fibers and surface irregularities during deposition. The sample in Figure 3b was produced with a high voltage (26.5 kV) and a low extrusion rate (1.5 mL/h). The high voltage stabilized the electrospinning jet, while the low extrusion rate supported the production of thinner and more uniform fibers. On the other hand, the sample shown in Figure 3c is the first NMC-doped TEOS fiber (TBN1), produced with a high extrusion rate (2.5 mL/h) and a voltage of 25 kV. The presence of NMC increased the conductivity of the solution, and the high extrusion rate led to uncontrolled fiber deposition, resulting in an uneven surface morphology. The second NMC-doped TEOS fiber sample (TBN2), shown in Figure 3d, was produced with a moderate voltage (24.5 kV) and a low extrusion rate (1.25 mL/h). The combination of low extrusion rate and relatively high voltage facilitated the formation of smoother fibers with improved structural uniformity.

In conclusion, higher voltage and lower extrusion rates generally resulted in fibers with more uniform morphology, while lower voltage or higher extrusion rates led to more irregular fiber deposition. Additionally, the presence of NMC influenced fiber morphology and showed the potential to reduce structural uniformity.

These observations align with the results presented by Geltmeyer et al. (2016), who demonstrated that similar electrospinning parameters—specifically, a voltage of 22.5 kV and a flow rate of 1 mL/h—facilitated the formation of bead-free fibers with consistent morphology. Their study also emphasized the critical role of controlled flow rate and sufficient voltage in obtaining fibers with desirable and homogeneous structural features [34]. Similarly, Klusoňová et al. (2025) reported the successful electrospinning of silica and NMC mixtures using parameters close to those in the present study (20 kV voltage, 1 mL/h flow rate, 15 cm distance), reinforcing the reliability and reproducibility of these electrospinning conditions for producing silica-based NMC fibers suitable for energy-related applications [35].

As illustrated in Figure 4, the untreated TEOS solution exhibits low viscosity (~0.006 Pa·s), and the viscosity remains nearly constant throughout the shear rate range. This behavior clearly indicates that the solution follows a Newtonian flow profile, where the viscosity does not vary with shear rate. Such behavior suggests weak intermolecular interactions and the absence of a polymeric network structure within the solution. According to the literature (e.g., [36]), solutions with a low degree of polymerization and minimal physical bonding tend to exhibit low viscosity and show little resistance change under shear stress. Therefore, the untreated TEOS solution demonstrates poor rheological characteristics and lacks sufficient molecular entanglement, making it unsuitable for consistent fiber formation via electrospinning.

Figure 5 presents the viscosity profile of the TEOS solution following thermal treatment at 80 °C for three hours, demonstrating an overall increase in viscosity and a more consistent response as shear rate rises. During thermal treatment, solvent evaporation and rearrangement of chemical bonds lead to densification of the molecular structure of the solution. Thermal treatment enhances the rate of TEOS hydrolysis and condensation reactions, which leads to an increased extent of polymerization in the solution. Consequently, the viscosity of the fluid increases, producing a more stable viscosity profile. This study demonstrates that thermal treatment modifies the rheological properties of the TEOS solution, making it more suitable for nanofiber production. After three hours of thermal treatment, the increased viscosity facilitates the alignment of polymer chains, enabling the formation of more uniform and continuous fibers. In electrospinning, the solution must possess a suitable viscosity range, and our findings show that heating the TEOS solution at 80 °C for three hours improves its spinnability for nanofiber fabrication. Loccufier et al. (2018, 2019) investigated the electrospinnability of TEOS-based sol–gels and identified that the most stable electrospinning range is between 120 and 200 mPa·s (0.12–0.2 Pa·s). They also demonstrated that electrospinning is feasible at viscosities up to 1.0 Pa·s [37,38]. In this context, the thermally treated TEOS-based sol–gel solution, with an average viscosity of 0.535 Pa.s, falls within the electrospinnable range and serves as a successful example of electrospinning performance. The optimization of thermal treatment time and temperature can be considered a critical parameter for enhancing the morphological and mechanical properties of the resulting nanofibers. Through controlled thermal treatment, sol–gel systems with viscosities in the desired range can be prepared, thereby ensuring stable processing conditions and the fabrication of uniform, structurally consistent nanofibers.

In this section, the FTIR spectra obtained from TEOS-based sol–gel solutions subjected to different aging durations at room temperature are presented in Figure 6. Measurements were performed at 0, 3, 5, 10, 15, 30, 45, and 70 min, as well as at 24 h. The obtained spectra reveal the key functional groups associated with the formation of the silica network.

The broad absorption band observed in the range of 3400–3200 cm⁻¹ corresponds to the stretching vibrations of –OH groups in the system. This band includes contributions from free water molecules, Si–OH groups, and the –OH groups of ethanol. As the aging time increased, no significant changes were observed in this band, except for slight attenuation. This finding indicates that hydrolysis and condensation reactions proceed very slowly at room temperature. Nevertheless, the band assignments are consistent with the values reported in the literature (Ibrahem and Ibrahem, 2014 [39]; Smith, 1964 [40]). The weak bands observed in the 2900–2850 cm⁻¹ range correspond to the C–H stretching vibrations of the ethyl groups in TEOS. Although these bands were initially more pronounced, they gradually weakened with increasing aging time, but did not completely disappear.

The strong peak around 1100 cm⁻¹, representing siloxane bonds, remained stable across all aging durations without showing a significant increase. Similarly, the Si–OH shoulder band near 950 cm⁻¹ persisted throughout the measurements. The symmetric Si–O–Si vibration at 800 cm⁻¹ and the rocking mode of bridging oxygen atoms at 450 cm⁻¹ were consistently observed in all samples. The presence of these bands confirms the silica-based structure and aligns with results reported in the literature (Swann et al., 2010 [41]; Hsiang et al., 2021 [42]).

Overall, the FTIR results demonstrate that aging TEOS-based solutions at room temperature for up to 24 h leads to only limited chemical changes. A partial reduction in –OH groups was observed, whereas a strengthening of Si–O–Si bonds could not be detected. This outcome indicates that sol–gel network formation under ambient conditions progresses very slowly, preventing the observation of distinct structural transformations in FTIR analysis. Nevertheless, the obtained spectra show a high level of consistency with the literature in terms of the positions of functional groups (Ibrahem and Ibrahem, 2014 [39]; Smith, 1964 [40]; Swann et al., 2010 [41]; Hsiang et al., 2021 [42]).

Figure 7 presents the confocal microscopy images of undoped TEOS-based and NMC-doped TEOS-based fibers. TB1 (a) and TB2 (b) represent undoped TEOS fibers, where differences in fiber diameter distribution and continuity can be observed. TBN1 (c) and TBN2 (d) represent NMC-doped TEOS fibers, with the presence of NMC particles clearly visible on the fiber surfaces. Scale bars: 20 µm for (a,b) and 50 µm for (c,d). These images were obtained to evaluate the morphological integrity of the fibers and to verify the distribution of NMC particles on the TEOS matrix, thereby enabling a direct comparison between undoped and NMC-doped samples.

Figure 8a shows the SEM image of the TB1 sample, which reveals a highly irregular fiber distribution, significant variations in fiber diameters, and a rough surface texture. In some regions, fibers appear to adhere to one another, and the differences in diameters are clearly noticeable. Additionally, the fibers are randomly oriented and deposited irregularly due to electrostatic forces. Confocal microscopy images confirm that the fibers are not aligned in a specific order, exhibit significant thickness variations across regions, and possess a wavy topography on their matte surfaces.

In Figure 8b, the SEM image of the TB2 sample presents a more homogeneous fiber distribution with more consistent diameters. The fibers are observed to be more uniformly aligned, surface roughness is reduced, and the overall morphology appears more controlled. Confocal microscopy images show that there is no significant variation in fiber diameters, fibers are more evenly distributed, and the matte surface is smoother. Compared to TB1, the TB2 sample demonstrates a more homogeneous and controlled fiber structure, indicating that the solution was more stable and the electrospinning parameters were more effectively optimized. This improvement is attributed to the application of a higher voltage of 26.5 kV, which contributed to maintaining the stability of the electrospinning jet. Similarly, Choi et al. (2003) reported that increasing the applied voltage during the electrospinning of TEOS-based solutions enabled the formation of finer and more uniform fibers, especially when voltages in the range of 12–16 kV were used instead of 10 kV [36]. In the same vein, Loccufier et al. (2018) demonstrated that applying high voltages in the range of 20–25 kV allowed the direct electrospinning of bead-free silica nanofibers with an average diameter of 316 nm from TEOS-based sol–gel systems [37]. These findings support the approach adopted in this study.

In Figure 8c, the SEM image of the TBN1 sample reveals distinct fiber breakages and bead formation. Fiber continuity was disrupted, and in some areas, complete fiber formation could not occur, resulting in bead-like structures instead. Such breakages and bead formation are typically associated with insufficient viscosity of the solution, poor dispersion of particles, or inadequate control of surface tension during electrospinning. Confocal microscopy images clearly highlight the irregularity of the TBN1 sample. Bead formation led to local fiber accumulation, resulting in larger structures. Furthermore, the disruption in fiber continuity at breakage points led to severe morphological irregularities. Considering that a TEOS+NMC-based solution was used, it is believed that the presence of NMC altered the rheological behavior of the solution, thereby hindering the formation of a stable jet during electrospinning, which in turn led to fiber breakage and bead formation.

In the SEM image of the TBN2 sample (Figure 8d), the fibers appear more uniformly distributed, with reduced bead formation and a more controlled morphological structure. Although variations in fiber diameter persist, they fall within a narrower range compared to the previous sample. This observation is supported by confocal microscopy images, which show that the fibers are more uniform, long, and continuous, with a decreased presence of beads and a generally more ordered structure. Despite the addition of NMC, the TBN2 sample exhibited a more consistent structure, which may be attributed to improved homogeneity of the solution or better optimization of electrospinning parameters. Following a detailed examination of the SEM images, the diameters of 100 fibers from each sample were measured. The SEM images were analyzed using ImageJ software (version 1.54g, Wayne Rasband and contributors, National Institutes of Health, Bethesda, MD, USA), and the obtained data were used to evaluate the fiber diameter distribution and structural consistency of each sample. The average fiber diameters of the TB1, TB2, TBN1, and TBN2 samples were measured as 2.060 µm, 0.345 µm, 2.471 µm, and 1.072 µm, respectively. In terms of diameter distribution, significant variations were observed in the TB1 and TBN1 samples, indicating inconsistency among the fibers. In the TB1 sample, some fibers were found to be very thick while others were extremely thin, resulting in an irregular diameter distribution. Similarly, notable variations in fiber diameter were also identified in the TBN1 sample. The addition of NMC to the TEOS sol led to an increase in fiber diameter; however, particularly in the TBN1 sample, the irregularity in diameter distribution was pronounced. On the other hand, the TB2 and TBN2 samples exhibited narrower diameter distributions, indicating better control during the electrospinning process. The TB2 sample had the smallest average fiber diameter at 0.345 µm, and the narrow distribution suggests that the solution had an appropriate viscosity and that the production process was carried out in a stable manner. In the TBN2 sample, the average fiber diameter was measured as 1.072 µm; although NMC was present, the diameter distribution was more homogeneous.

Overall, TB1 and TBN1 samples exhibited irregular morphologies, whereas TB2 and TBN2 samples presented more homogeneous structures. Among the pure TEOS-based fibers, the TB2 sample showed the smallest diameter and the most structurally consistent fibers, while the TB1 sample displayed significant diameter variations. Among the TEOS+NMC-based fibers, the TBN1 sample had the most irregular diameter distribution, while the TBN2 sample exhibited a more controlled structure. Although the addition of NMC increased fiber diameters, it was observed in the TBN2 sample that more homogeneous fibers could be produced when the production conditions were effectively optimized. These results indicate that fiber diameter control is highly dependent on the selected electrospinning parameters, and that process optimization plays a critical role in achieving structural consistency. The observed morphological irregularities are thought to result from the instability of the solution during electrospinning or the inability of the process parameters to adequately control fiber formation.

EDS analyses were performed to assess the chemical composition through point analysis of the fiber structures. For both pure TEOS-based fibers and TEOS-based fibers containing NMC, several points were selected from the SEM micrographs. Among these, the regions identified as Spectrum 2 were chosen for detailed analysis, as they were considered to best represent the characteristic structure of each sample. These spectra were analyzed to reveal the overall chemical characteristics of the materials.

In the pure TEOS-based fiber shown in Figure 9, the EDS analysis of the Spectrum 2 region revealed oxygen (O) and silicon (Si) as the primary elements. The oxygen content was measured as 53.7%, while the silicon content was found to be 46.3%. These results indicate that TEOS underwent hydrolysis and initiated the formation of Si-O-Si bonds following a preliminary heat treatment at 80 °C for three hours, which rendered the solution suitable for electrospinning. Although the resulting fibers did not form a fully condensed silica structure, the dominance of Si and O in the EDS results demonstrates that the material exhibits the characteristics of a condensed silica precursor. The carbon (C) and gold (Au) signals were excluded from the analysis, as their presence could result from organic residues or sample coating. These findings align with those previously reported by Shahhosseininia (2018) for silica-based fibers without additional treatment [43]. In the NMC-doped TEOS-based fiber shown in Figure 10, Spectrum 2 was selected for a detailed elemental analysis. According to the EDS results, the fiber composition consists of 36.1 wt% nickel (Ni), 22.2 wt% silicon (Si), 17.7 wt% manganese (Mn), 15.0 wt% oxygen (O), 7.4 wt% cobalt (Co), and 1.5 wt% aluminium (Al). This elemental distribution confirms that the main components of the NMC structure (LiNiMnCoO₂) were successfully incorporated into the TEOS-based fiber matrix. As noted above, carbon (C) and gold (Au) were excluded from the evaluation due to their association with external sources or sample preparation. The EDS analysis clearly demonstrated the compositional difference between the NMC-doped TEOS-based fibers and the undoped counterparts, verifying the successful integration of NMC into the electrospun fiber structure.

X-ray diffraction (XRD) analysis was conducted on fibers that underwent heat treatment at 650 °C for one hour. Figure 11 presents the XRD patterns of undoped TEOS fibers and NMC-doped TEOS fibers. In the XRD pattern of the undoped TEOS-based fiber (black curve), a broad, low-intensity peak is observed around 2θ ≈ 23°, which is characteristic of an amorphous silica structure. This indicates that TEOS underwent condensation during electrospinning, resulting in the formation of an amorphous S-O-Si network. A partially sharp peak appearing around 2θ ≈ 45° is attributed to the Fe sample holder, not to the fiber material itself. In the XRD pattern of the NMC-doped TEOS fiber (red curve), relatively more defined yet still broad diffraction peaks are observed compared to the undoped sample. These reflections indicate the formation of weak or early-stage crystalline phases. The peak around 2θ ≈ 19° is marked in green and corresponds to the Ni(NO₃)₂·2(H₂O)₂ phase, as indicated in the figure. The blue-marked peaks around 2θ ≈ 38° and 44° are associated with the SiO₂ phase. The lack of sharpness in these peaks suggests that a fully developed crystalline structure has not formed; however, the incorporation of elements from the NMC additive appears to have triggered partial crystallization within the system. These findings indicate that, compared with undoped TEOS fibers, the addition of NMC results in a moderate increase in crystallinity.

Thermal stability and decomposition behavior of electrospun undoped TEOS and NMC-doped TEOS-based fibers were evaluated by TG/DTA analysis, as shown in Figure 12 and Figure 13.

In Figure 12, the TG-DTA analysis of undoped TEOS fibers shows an endothermic peak in the DTA curve accompanied by a 10% mass loss in the TG curve between 0 and 200 °C. This mass loss is attributed to the removal of physically adsorbed water and residual ethanol. Between 200 and 600 °C, two distinct exothermic events were detected in the DTA curve, corresponding to an additional mass loss of approximately 8%, which is associated with the decomposition of residual organic constituents. The absence of significant mass loss above 600 °C indicates that the organic components were almost completely removed, resulting in the formation of a thermally stable silica network. The total mass loss of about 18% reflects the low organic content and demonstrates the effectiveness of the applied thermal treatment. These findings provide important insights into the appropriate heat-treatment conditions required to obtain fully dried and thermally stable silica fiber mats and are consistent with the thermal behavior reported by Frontera et al. (2019) [44].

Figure 13 presents the TG-DTA results of NMC-doped TEOS-based fibers, obtained to investigate their thermal stability and decomposition behavior. In this sample, an initial mass loss of approximately 8% was recorded near 200 °C, attributed to the removal of physically adsorbed water, ethanol, and other volatile components. An endothermic peak observed at around 130 °C confirmed this evaporation process. Between 200 and 650 °C, the DTA curve exhibited a broad and irregular exothermic region, corresponding to a mass loss of about 4%. This stage is mainly associated with the decomposition of partially cross-linked silanol groups and residual organic constituents (including TEOS) within the matrix, and may also involve oxidative rearrangements of the Ni, Mn, and Co components in the NMC structure.

A distinct exothermic peak appeared at around 750 °C, accompanied by a small but noticeable mass loss of about 1% in the TG curve. This peak is most likely related to crystallization or phase transitions of the metal oxide phases (particularly NiO and Co₃O₄) present in the NMC content. Finally, in the 900–1000 °C range, the DTA curve exhibited a secondary exothermic peak without a corresponding mass loss in the TG curve. This phenomenon is more plausibly attributed to structural reorganization or secondary phase transitions into more stable crystalline configurations rather than complete crystallization. The total mass loss was determined to be approximately 13%, and the cessation of mass loss above 750 °C indicates that organic residues within the fiber matrix were almost completely eliminated, leading to the formation of a thermally stable inorganic fiber structure. The relatively low total mass loss observed in TG analysis highlights the key advantages of polymer-free SiO₂ fibers, namely their high purity and enhanced thermal stability.

This analysis provides important insights into the thermal stability of both undoped and NMC-doped TEOS-based fibers, identifying the decomposition temperatures of organic residues. These findings may serve as a valuable reference for defining appropriate pre-calcination temperatures prior to subsequent sintering or potential electrochemical applications.

Electrochemical impedance spectroscopy (EIS) measurements of the undoped TEOS fiber clearly demonstrate that the system behaves like an ideal capacitor. As seen in Figure 14, a nearly straight line extending along the negative imaginary (Z″) axis is observed. This indicates that the charge transfer resistance is negligible and that the system lacks electronic conductivity; as in an ideal capacitor, the real part of the impedance is zero, and the current arises not from electron flow between electrodes but from charge accumulation. These findings are in full agreement with the ideal capacitive behavior described by Bardini (EIS 101, Section 2.6.2) [45,46]. These results clearly indicate that the undoped TEOS fibers exhibit a purely dielectric character, with no observable electron transfer or active ion transport mechanism. From the perspective of battery applications, this structure cannot be directly utilized as an active electrode material due to its negligible electronic conductivity and absence of charge transfer processes. However, its high impedance and stable structure make it suitable for use as an insulating interlayer, separator, electrode coating, or passive interface layer in solid-state batteries. Furthermore, the well-known properties of silica-based compounds and TEOS, such as high mechanical strength, chemical stability, and resistance to high temperatures, suggest that these capacitive structures have potential as dielectric support materials in applications requiring high thermal and chemical resistance.

The Nyquist plot of the NMC-doped TEOS fiber, shown in Figure 15, demonstrates a clear deviation from ideal capacitive behavior, revealing the simultaneous presence of both resistive and capacitive characteristics. While the undoped TEOS-based fiber exhibits a nearly vertical line along the imaginary axis (Z″), the NMC-doped sample shows a curve that bends toward the real axis (Z′). This indicates the presence of a measurable real component of impedance (Z′) and a non-negligible charge transfer resistance. The resulting impedance curve implies a shift away from ideal capacitive behavior. Although a complete semicircle is not formed, the observed curvature implies the initiation of charge transfer processes and the emergence of electronic conductivity pathways with the addition of NMC. Compared with the undoped TEOS-based fiber, the real part of the impedance (Z′) increased significantly. Meanwhile, the imaginary part (Z″) approaches a plateau in the high-frequency region, reflecting partial conductivity and capacitive charge accumulation. This impedance behavior indicates the onset of charge transport and partial conductivity mechanisms, which have also been observed in lithium-ion batteries (Murbach et al., 2018) [46]. Such behavior aligns with impedance profiles commonly associated with electrochemical systems involving charge transfer and double-layer formation, as reported similarly by Murbach and colleagues in their work on lithium-ion cells. In conclusion, incorporation of NMC into the TEOS fiber matrix imparted resistive characteristics to the system and enabled limited charge transport mechanisms. This represents a shift from purely dielectric behavior toward a more conductive electrochemical response. Although the system is not yet functioning as a fully active electrode material—evidenced by the lack of a distinct semicircle in the Nyquist plot—the decrease in impedance and the increase in conductivity offer a promising foundation for potential applications in energy storage.

4. Conclusions

• This study successfully produced TEOS-based fibers using the electrospinning method with solutions free of carrier polymers, instead of TEOS-based fibers, which are mostly produced with polymer additives.
• Electrospinning parameters played a decisive role in fiber morphology; high voltage and low flow rate promoted the formation of smooth, uniform fibers, whereas low voltage, high flow rate, and the presence of additives led to morphological irregularities and structural defects.
• Thermal analyses of both undoped and NMC-doped TEOS fibers revealed that organic components were largely eliminated, resulting in thermally stable inorganic structures suitable for high-temperature applications.
• Compared with polymer-assisted electrospinning methods, the polymer-free approach employed in this study significantly reduced residual organic components, leading to high-purity SiO₂ fibers with enhanced thermal stability. This characteristic is particularly advantageous for applications that demand chemical and thermal resilience.
• Electrochemical impedance spectroscopy (EIS) results clearly demonstrated that the undoped TEOS fibers behave like ideal capacitors.
• Although undoped TEOS fibers cannot function as active electrode materials, their high impedance and structural stability make them suitable for use as insulating interlayers, separators, electrode coatings, or passive interface layers in solid-state batteries.
• The NMC-doped TEOS fibers exhibited a clear deviation from ideal capacitive behavior, displaying both resistive and capacitive characteristics. This behavior suggests that the system no longer acts as an ideal capacitor; instead, it begins to exhibit characteristics of a partially conductive electrochemical structure.
• Although the NMC-doped TEOS fibers do not yet exhibit the full characteristics of an active electrode material, the observed reduction in impedance and increase in conductivity provide a promising basis for future application in energy-storage systems.