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 (Mw = 100 kg/mol) and low molecular weight (Mw = 0.35 kg/mol)─were blended with LiTFSI and NaTFSI salts to explore ion transport mechanisms. In the high-Mw PEO, where ion hopping dominates, smaller Li+ ions exhibit higher conductivity (σLiTFSI > σNaTFSI). In contrast, the low-Mw PEO, where ion diffusion is the primary mechanism, shows higher conductivity for larger Na+ ions (σNaTFSI > σLiTFSI). 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 σNaTFSI ≈ 4 × 10-4 S/cm, meeting the requirements for practical applications. These findings highlight the potential of low-Mw PEO and Na-based electrolytes for the development of efficient Na-ion batteries.