Ion-conductive vs. non-ion-conductive ceramic fillers in silane-linked polyethylene oxide-based composite polymer electrolytes with high room-temperature ionic conductivity

Polyethylene oxide (PEO)-based polymer electrolytes, despite their cost-effectiveness and ease of processing, suffer from low ionic conductivity at lower temperatures due to the semi-crystalline nature of PEO. Incorporating ceramic filler particles into the polymer matrix offers a potential solution by disrupting its rigid crystalline structure, thereby improving the flexibility of the polymer chains. However, the Li ion conduction pathway within these composite polymer electrolytes (CPEs) remains predominantly within the polymer matrix if the filler particles are only physically mixed. The surface modification of filler particles can improve the interfacial compatibility and ionic conductivity. In this work, two types of filler particles, passive ZrO₂ and active Li₇La₃Zr₂O₁₂ (LLZO), are compared and incorporated into PEO–polyethylene glycol (PEG)–lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) CPEs. The surface of the filler particles is functionalized with a silane ligand ((3-glycidyloxypropyl)trimethoxysilane (GPTMS)) prior to their integration into the PEO matrix. This modifies the interfacial properties between the polymer and the filler particles, hence influencing the ionic conductivity. The functionalized ZrO₂ fillers enhance the ionic conductivity of the CPEs by reducing the crystallinity of PEO. The PEO–PEG–LiTFSI CPE with 15 vol% of GPTMS–ZrO₂ achieved an ionic conductivity of 6.66 × 10⁻⁴ S cm⁻¹ at 20 °C, which is significantly higher than that of the standard PEO–LiTFSI (9.26 × 10⁻⁶ S cm⁻¹). Additionally, coupling GPTMS to PEO chains without the introduction of filler particles also improved the ionic conductivity, while the incorporation of functionalized LLZO fillers does not, which is attributed to a LiCO₃ passivation layer. The results suggest a viable strategy to overcome the inherent limitations of PEO electrolyte, thus offering valuable insights into the design and optimization of CPEs for practical applications.