Ion‑Conductive Epoxy Polymer Composites with Lithium Perchlorate: Structural Insights and Conductivity Enhancement

Abstract

We synthesized hybrid amorphous polymers from diglycide aliphatic ester of polyethylene glycol (DEG‑1) cured with polyethylene polyamine (PEPA) and incorporated lithium perchlorate (LiClO₄). Differential scanning calorimetry, wide‑angle X‑ray diffraction, infrared spectroscopy, scanning electron microscopy, and optical microscopy revealed that LiClO₄ forms donor‑acceptor coordination complexes with the epoxy chains, raising the glass transition temperature (T_g) while also producing inorganic‑looking microinclusions.

Background

Liquid electrolytes dominate current lithium‑ion batteries, delivering ionic conductivities of 10⁻³–10⁻² S cm⁻¹ at room temperature [1, 2]. Operating at elevated temperatures would eliminate the need for extensive cooling systems, yet liquid electrolytes degrade rapidly and can form lithium dendrites that threaten safety and cycle life [3, 4]. Solid polymer electrolytes (SPEs) are a promising alternative because they are non‑flammable, leak‑free, and compatible with a wide range of electrodes [5–10].

Polyethylene oxide (PEO) is the most studied oligomer for SPEs due to its ability to coordinate Li⁺ ions, but its crystalline domains limit room‑temperature conductivity [11–17]. Incorporating high concentrations of lithium salts into amorphous polymer matrices can enhance ion transport, yet excessive salt can form ion complexes that reduce mobility [6, 9, 10, 17].

Aliphatic epoxy oligomers such as DEG‑1 share the ether backbone of PEO but remain fully amorphous, making them suitable for dissolving large amounts of LiClO₄ while preserving processability. This study focuses on how varying LiClO₄ content (0–50 phr) influences the structure and ionic conductivity of DEG‑1/PEPA composites.

Methods

Materials and Synthesis

DEG‑1 and LiClO₄ were vacuum‑drying at 80 °C for 24 h before mixing. Solutions of DEG‑1–LiClO₄ were prepared with salt loadings from 0 to 50 phr; 10 phr PEPA served as hardener. The composites were cured under standard conditions.

Thermal Characterization

DSC (TA Instruments DSC Q2000) measured −70 °C to +150 °C at 10 °C min⁻¹. T_g was extracted from the second heating scan with ±1 °C precision.

Electrical and Dielectric Measurements

Broadband dielectric analysis (Novocontrol Alpha with Quatro Cryosystem) covered 0.1 Hz–10⁷ Hz and −60 °C to +200 °C. Samples (20 mm diameter, 0.5 mm thick) were aluminum‑coated under vacuum; a 0.5 V bias was applied.

Structural Analyses

Wide‑angle X‑ray diffraction (WAXS) employed a DRON‑4.7 diffractometer with CuK_α radiation (λ = 1.54 Å). Infrared spectra were recorded with a Bruker Tensor‑37 FT‑IR (600–3800 cm⁻¹) with <2% error. Reflective optical microscopy (ROM) used a Unicorn NJF 120A polarizer (0°–90°). Scanning electron microscopy (SEM) (JEOL 100‑CX II) combined with EDX (JEOL JSM‑35CF, INCA Energy‑350) provided elemental maps; the maximum probe size was 2 µm and the analytical error 0.1 %. All structural studies were performed at 20 ± 2 °C.

Results and Discussion

DSC data for 0–20 phr LiClO₄ were previously reported [22]. Extending the salt loading to 50 phr yielded a linear rise in T_g from −10 to 64 °C (Figure 1a), attributable to electrostatic coordination between Li⁺ and the ether oxygens in DEG‑1, which stiffens the polymer matrix.

Figure 1b shows conductivity (σ) versus LiClO₄ content. At 60 °C, σ peaks at 10 phr; by 20 phr it matches the pure DEG‑1 value. At 200 °C, σ is three orders of magnitude higher, with a maximum at 15 phr. The dual influence of increased charge carriers and reduced chain mobility explains this trend: higher temperatures enhance segmental motion, mitigating the mobility loss caused by coordination.

WAXS patterns (Figure 2) confirm that all composites remain amorphous. However, a weak diffraction peak near 2θ ≈ 12.2° emerges with LiClO₄ addition, indicating short‑range ordering of Li⁺–ether complexes (d ≈ 4.30 Å). This observation corroborates the DSC‑derived T_g increase.

Infrared spectroscopy (Figure 3) reveals the disappearance of epoxy ring vibrations, confirming complete curing. Bands at 1300–1520 and 1000–1190 cm⁻¹ shift to lower frequencies with higher salt loadings, signifying coordination of Li⁺ with both ether and amine groups. The LiClO₄ characteristic 1637 cm⁻¹ band vanishes in the composites, indicating full ion dissociation.

New absorption at 864 cm⁻¹ appears with 5 phr LiClO₄ and persists up to 50 phr, a signature of Li–amino complexes. These bands disappear when the films are powdered and mixed into KBr, confirming their weak coordination nature.

ROM (Figure 6) and SEM (Figure 7) images reveal dispersed inclusions ranging from 2 to 20 µm, increasing in number and size with salt content. EDX maps (Figure 8) show these inclusions are rich in oxygen and chlorine, consistent with LiClO₄ aggregation, and depleted in carbon relative to the polymer matrix.

Elemental analysis of the original LiClO₄ confirms a 41.61 wt% Cl and 58.39 wt% O composition; lithium was not quantified. The composite containing 20 phr LiClO₄ exhibited surface elemental distribution of 51.57 wt% C, 43.79 wt% O, and 4.64 wt% Cl (Figure 9), reinforcing the inorganic nature of the inclusions.

Conclusions

LiClO₄ addition to DEG‑1/PEPA composites forms Li⁺–ether coordination complexes, linearly raising T_g and moderating segmental mobility. Conductivity peaks at 15 phr LiClO₄ at 200 °C, driven by a balance between increased carrier density and chain dynamics. IR spectroscopy and morphological studies confirm the formation of Li–amide and Li–ether complexes and the presence of inorganic inclusions spanning nanometer to micron scales.

Abbreviations

DEG‑1

Epoxy oligomer of diglycide aliphatic ester of polyethylene glycol

DSC

Differential scanning calorimetry

IR

Infrared spectroscopy

LiClO₄

Lithium perchlorate salt

PEO

Polyethylene oxide

PEPA

Polyethylene polyamine

ROM

Reflective optical microscopy

SEM

Scanning electron microscopy

SPE

Solid polymer electrolyte

T_g

Glass transition temperature

TOM

Transmission optical microscopy

WAXS

Wide‑angle X‑ray spectroscopy