Cation Chemistry and Molecular Weight Effects on the Ion Conductivity in PEO-based Electrolytes

This study investigates the fundamental influence of cation chemistry on the ionic conductivity of PEO-based electrolytes, with implications for advancing polymer electrolyte design. Two PEO systems─high molecular weight (<i>M</i>_w = 100 kg/mol) and low molecular weight (<i>M</i>_w = 0.35 kg/mol)─were blended with LiTFSI and NaTFSI salts to explore ion transport mechanisms. In the high-<i>M</i>_w PEO, where ion hopping dominates, smaller Li⁺ ions exhibit higher conductivity (σ_LiTFSI > σ_NaTFSI). In contrast, the low-<i>M</i>_w PEO, where ion diffusion is the primary mechanism, shows higher conductivity for larger Na⁺ ions (σ_NaTFSI > σ_LiTFSI). In the former, rheology measurements indicate that larger Na⁺ cations form more transient EO:Na⁺ contact, hindering cation hopping and reducing conductivity. In the latter, the stronger EO:Li⁺ interactions lead to a larger hydrodynamic radius and slower diffusion. Notably, PEO-0.35K:NaTFSI exhibits a room-temperature conductivity of σ_NaTFSI ≈ 4 × 10^-4 S/cm, meeting the requirements for practical applications. These findings highlight the potential of low-<i>M</i>_w PEO and Na-based electrolytes for the development of efficient Na-ion batteries.