Dielectric relaxation spectroscopy for the characterization of ion transport in solid polymer electrolytes in Li-ion cells

Solid polymer electrolytes (SPEs) have emerged as promising candidates for lithium-ion batteries. However, their practical application has been hindered by the limited ionic conductivity (< 10-3 S/cm), attributed primarily to the slow segmental dynamics of the polymer in the solid state. To elucidate the Li+ ion transport mechanism in SPEs, dielectric relaxation spectroscopy (DRS) can be used to measure both ionic conductivity and segmental dynamics simultaneously, as well as their temperature dependences. Various approaches have been explored to improve ionic conductivity, including the use of polymers with low glass transition temperature, high salt solubility, and an increased fragility. Additionally, single-ion conducting polymer electrolytes, polymer-in-salt systems, polymer blends, and composite SPEs have shown potential for achieving higher ionic conductivity. A novel approach focused on decoupling ion transport from polymer segmental relaxation and creating ion channels, such as super-ionic glasses, is currently under investigation to improve ionic conductivity at room temperature. In this review, we highlight recent DRS investigations that have provided insights into the Li+ ion transfer mechanism and its correlation with the improvement in ionic conductivities.