Microstructural and Spectral Characterization of ZrO₂-Doped PEO/PMMA Nanocomposite Polymer Electrolytes ^†

Abstract

Blended nanocomposite solid polymer electrolytes are gaining considerable attention as next-generation materials for use in flexible lithium-ion battery systems. These materials help ensure a more uniform distribution of lithium ions at the electrode–electrolyte interface, contributing to the development of a stable interfacial layer that mitigates lithium dendrite formation. In this study, solid polymer electrolytes were synthesized using a binary polymer matrix composed of polyethylene oxide (PEO) and polymethyl methacrylate (PMMA), with lithium iodide (LiI) as the ionic salt. Zirconium dioxide (ZrO₂) nanoparticles were introduced as nanofillers in varying concentrations to investigate their influence on the physical and functional characteristics of the polymer matrix. Characterization was carried out using Scanning Electron Microscopy (SEM), Fourier Transform Infrared Spectroscopy (FTIR), and X-ray Diffraction (XRD). SEM images indicated that ZrO₂ nanoparticles remained well-dispersed up to 3 wt%, while higher loadings showed slight agglomeration. FTIR analysis revealed noticeable changes in absorption bands, suggesting strong interactions among polymer chains and the nanofillers. XRD data confirmed the semi-crystalline behavior of the PEO/PMMA blend system. The inclusion of ZrO₂ nanofillers enhanced the structural integrity and ionic conductivity of the polymer matrix, making them promising candidates for applications in electrochemical energy storage and advanced material interfaces. The systematic incorporation of ZrO₂ nanofillers into the PEO/PMMA matrix significantly improved the microstructural uniformity, polymer–filler interactions, and ionic transport behavior of the solid polymer electrolytes.

1. Introduction

The rapidly increasing demand for safer, flexible and high-performance lithium-ion batteries has stimulated extensive exploration of solid polymer nanocomposite electrolytes as effective alternatives to predictable liquid electrolytes [1]. Despite their widespread use due to high ionic conductivity, liquid electrolytes exhibit major shortcomings, notably electrolyte leakage and safety concerns arising from their flammable nature and poor interface stability with electrodes. These issues not only compromise battery safety but also accelerate lithium dendrite growth, which can result in internal short circuits and device failure [2]. Solid polymer electrolytes offer improved safety, mechanical flexibility and better electrode–electrolyte contact, making them attractive for next-generation energy storage systems [3]. Among various polymer hosts, polyethylene oxide has been extensively studied because of its excellent capability to solubilize lithium salts via coordination interactions involving Li⁺ and ether oxygen atoms, forming ion-conducting complexes [4]. However, the high degree of crystallinity of PEO at room temperature significantly restricts polymer chain mobility, leading to low ionic conductivity. To overcome this limitation, blending PEO with amorphous polymers such as polymethyl methacrylate has proven effective in reducing crystallinity while enhancing mechanical strength and thermal stability. The resulting semi-crystalline polymer matrix provides improved ion transport pathways through its amorphous regions. Furthermore, the incorporation of inorganic nanofillers into polymer blends has emerged as an efficient strategy to enhance electrolyte performance. Particularly, zirconium oxide nanoparticles have attracted considerable attention due to their high thermal stability, dielectric constant and strong interaction with polymer chains. These nanofillers disrupt polymer packing, increase free volume, promote lithium salt dissociation, and improve interfacial stability, thereby enhancing ionic conductivity [5]. Characterization methods including Scanning Electron Microscopy, Fourier Transform Infrared Spectroscopy and X-ray Diffraction played a crucial role in evaluating the structural and chemical performance of these nanocomposite polymer electrolyte films. SEM provides information on surface morphology and nanoparticle dispersion, FTIR identifies molecular-level interactions between polymers, salt and nanofillers while XRD reveals changes in crystallinity that directly influence ion transport efficiency [6]. Consequently, PEO-PMMA-based nanocomposite solid polymer electrolyte systems incorporating ZrO₂ nanoparticles represent a promising approach for developing safe, stable and high-performance solid-state lithium-ion batteries.

2. Analytical Attributes

2.1. Reagent-Assisted Material Synthesis

Reagents like poly(methyl methacrylate), possessing a molecular weight of 9.96 × 10⁵ g mol⁻¹, and poly(ethylene oxide) with a molecular weight of 5 × 10⁵ g mol⁻¹, lithium iodide (LiI) and Zirconium dioxide (ZrO₂) nanoparticles with dispersion below 100 nm were procured from Sigma Aldrich. All reagents were used without further purification.

2.2. Fabrication of Solid Polymer Nanocomposite Electrolyte Films

Solid nanocomposite polymer electrolyte films were synthesized using the conventional solution casting approach [7]. Initially, the required quantities of PEO (0.2 g) and PMMA (0.1 g) were dried in a vacuum oven at 373 K for 15 min to eliminate absorbed moisture. The dried polymers were dissolved in acetone (30 mL) and stirred continuously to obtain a clear and uniform polymer solution. The lithium salt of 10 wt% with respect to the total polymer blend (PEO/PMMA) was independently dried under vacuum and then slowly incorporated into the polymer blend while stirring. The mixture was further stirred to promote effective interaction between the polymer chains and lithium ions. Subsequently, ZrO₂ nanoparticles were added at loadings of 3 wt% and 5 wt% calculated based on the total weight of the polymer matrix into the polymer–salt solution and dispersed by prolonged magnetic stirring for 2 h to ensure homogeneity [8]. The resulting solution was poured into a clean Petri dish and dried under ambient conditions to facilitate solvent evaporation. After complete drying, flexible and free-standing nanocomposite solid polymer electrolyte films were obtained and preserved in a desiccator to avoid moisture contamination before characterization.

2.3. Characterization

Surface morphology and microstructural features were studied using a Hitachi S-3400N SEM under cryogenic conditions in liquid nitrogen [9]. FTIR spectrum was approved using a Bruker Tensor 27 spectrometer to investigate molecular interactions and complex formation [10]. The crystalline nature of the nanocomposite solid polymer electrolyte films was investigated by XRD with a Bruker diffractometer utilizing radiation having the wavelength of λ = 1.54 Å in the 2θ ranging from 10° to 80° [7].

3. Results and Discussion

3.1. Microstructural Insights from SEM Investigation

SEM analysis exposed in Figure 1a–c reveals the progressive modification in the surface morphology of PEO/PMMA/LiI-based electrolytes with increasing ZrO₂ content. The filler-free electrolyte exhibits compact and closely packed surface domains, indicating residual crystalline ordering that can impede ion mobility [11]. Incorporation of 3 wt% ZrO₂ leads to a more uniform and smoother surface, confirming effective nanoparticle dispersion and enhanced amorphous character of the polymer matrix which supports continuous ion transport pathways [12]. However, at 5 wt% ZrO₂ loading, surface irregularities and localized particle aggregation become evident, reducing polymer chain flexibility and partially obstructing ionic conduction [13]. These morphological observations establish 3 wt% ZrO₂ as the optimal filler concentration for achieving improved ionic conductivity in the nanocomposite electrolyte system [14].

3.2. FTIR Profiling of Molecular Structure in Solid Polymer Electrolyte Films

The FTIR spectra of PEO/PMMA/LiI and 3 wt% ZrO₂-filled PEO/PMMA/LiI films are presented in Figure 2. The pristine electrolyte exhibits characteristic absorption bands corresponding to both PEO and PMMA components. The band observed near ~852 and ~1452 cm⁻¹ is attributed to the C–C symmetric stretching and –CH₂ scissoring vibration of PEO [15]. Upon incorporation of 3 wt% of ZrO₂ nanoparticles, this band gets slightly shifted to ~854 and ~1461 cm⁻¹, suggesting possible interactions between ZrO₂ filler and ether oxygen groups of PEO. Peaks at ~963 and ~1092 cm⁻¹ endorsed to -CH₂ rocking and -C-O-C stretching mode of PEO were repositioned to ~946 and ~1090 cm⁻¹ [8,15]. The vibrational band observed at ~1967, ~2860 and ~2884 cm⁻¹ is attributed to asymmetric stretching and -C-H stretching mode of PEO was repositioned to ~1984, ~2851 and ~2911 cm⁻¹ respectively [16,17]. The characteristic vibrational band observed at ~1727 cm⁻¹ associated with C=O stretching of carbonyl group of PMMA polymer gets moved to ~1744 cm⁻¹ [8]. Dual bands observed at ~1238 and ~1281 cm⁻¹ correspond to -C-C-O in the ester group, and (C-C-O) stretching vibrations in the methyl carbonyl group [18,19] of PMMA polymer are altered to single band ~1272 cm⁻¹ after the incorporation of ZrO₂ nanofiller. This may be due to the strong interaction between polymer, salt and filler which may cause reduced crystallinity. The band at ~1573 cm⁻¹ representing the coordination of lithium ions with the carbonyl (C=O) group of the PMMA matrix [20] gets shifted to ~1581 cm⁻¹ in the filler loaded system, which indicates stronger interaction and possible modification of the local environment due to presence of the filler. It should be noted that absorption bands in the range of 1555–1580 cm⁻¹ may also correspond to the asymmetric stretching vibration of carboxylate groups (ν_asym COO⁻), while symmetric stretching modes typically appear near ~1370 cm⁻¹. Therefore, the band observed in this region may also include contributions from carboxylate-type coordination formed due to strong interactions between lithium ions, the PMMA matrix, and the filler surface. This indicates that the interaction mechanism in the composite electrolyte may involve complex polymer–salt–filler coordination rather than simple Li⁺–carbonyl interaction alone. Peaks at ~1341 cm⁻¹ consigned to –CH₂ deformation of PMMA disappear in the complex after the inclusion of ZrO₂ filler whereas the band at 3452 cm⁻¹ attributed to the -OH group moved to ~3469 cm⁻¹ [21]. Overall, the observed peak shifts and intensity variations suggest possible polymer–salt–filler interactions and when considered alongside SEM and XRD analyses, it provides supportive evidence of structural modifications induced by ZrO₂ incorporation.

3.3. Structural Investigation by XRD

The crystalline structure of PEO/PMMA/LiI polymer electrolyte with and without ZrO₂ nanofiller (3 and 5 wt%), was investigated using X-ray diffraction, as shown in Figure 3. The filler-free electrolyte displays distinct diffraction peaks around 2θ ≈ 19.1° and 23.4°, characteristic of the semi-crystalline nature of PEO, while the presence of a broad halo at lower angles corresponds to the amorphous PMMA phase, which partially suppresses long-range ordering [4]. The incorporation of 10 wt% LiI did not result in the appearance of additional crystalline peaks or distinct phase reflections in the XRD patterns. They showed the peaks corresponding to PEO and PMMA only which indicates that no detectable phase separation occurred within the investigated composition range. Upon the addition of ZrO₂ nanoparticles, these PEO-related peaks become progressively broader, less intense and shifted to 19.3° and 23.5°, indicating disruption of crystalline domains due to strong interactions among polymer chains, lithium ions and the ceramic filler ZrO₂ [22]. This structural modification is accompanied by the appearance of weak diffraction peaks at approximately 2θ ≈ 28.4° and 31.7°, confirming the successful incorporation of crystalline ZrO₂ within the polymer matrix. Although the electrolyte containing 3 wt% ZrO₂ exhibits the greatest reduction in crystallinity, this presents an optimal balance between amorphous content and uniform filler dispersion. This structural balance is favorable for enhanced segmental motion and continuous ion-transport pathways, which explains the superior ionic conductivity observed for the 3 wt% ZrO₂-based electrolyte films [14]. Addition of 5 wt% ZrO₂ shows the increase in intensity of peak which indicates the crystalline nature. Overall, the XRD results demonstrate that controlled incorporation of ZrO₂ effectively tailors the polymer structure, making the nanocomposite electrolyte films suitable for solid-state electrochemical applications.

4. Conclusions

In this study, PEO/PMMA/LiI-based solid polymer nanocomposite electrolyte systems incorporating ZrO₂ nanoparticles were successfully prepared via the solution casting method. Their structural, morphological, and spectroscopic properties were systematically investigated using XRD, FTIR and SEM analyses. SEM observations demonstrated that the addition of 3 wt% ZrO₂ resulted in a relatively smooth and homogeneous morphology with uniform filler dispersion, whereas higher filler loading leads to particle agglomeration and surface irregularities. FTIR results confirmed strong coordination among Li⁺ ions, polymer functional groups and ZrO₂ nanoparticles, suggesting the formation of polymer–salt–filler complexes. XRD results revealed a reduction in the characteristic crystalline peaks of PEO upon ZrO₂ incorporation, indicating an increase in the amorphous nature of the polymer matrix. These structural and morphological modifications suggest that controlled addition of ZrO₂ influences the microstructural properties of the polymer blend. Among the investigated compositions, the sample containing 3 wt% ZrO₂ exhibited improved structural uniformity and enhanced amorphous character compared to other compositions. The present findings provide insight into the role of nanofiller incorporation in modifying polymer blend characteristics, and further electrochemical investigations are necessary to evaluate their suitability for solid-state electrolyte applications.