Single-Ion versus Dual-Ion Conducting Electrolytes: The Relevance of Concentration Polarization in Solid-State Batteries

Lithium batteries with solid polymer electrolytes (SPEs) and mobile ions are prone to mass transport limitations, that is, concentration polarization, creating a concentration gradient with Li⁺-ion (and counter-anion) depletion toward the respective electrode, as can be electrochemically observed in, for example, symmetric Li||Li cells and confirmed by Sand and diffusion equations. The effect of immobile anions is systematically investigated in this work. Therefore, network-based SPEs are synthesized with either mobile (dual-ion conduction) or immobile anions (single-ion conduction) and proved via solvation tests and nuclear magnetic resonance spectroscopy. It is shown that the SPE with immobile anions does not suffer from concentration polarization, thus disagreeing with Sand and diffusion assumptions, consequently suggesting single-ion (Li⁺) transport via migration instead. Nevertheless, the practical relevance of single-ion conduction can be debated. Under practical conditions, that is, below the limiting current, the concentration polarization is generally not pronounced with DIC-based electrolytes, rendering the beneficial effect of SIC redundant and DIC a better choice due to better kinetical aspects under these conditions. Also, the observed dendritic Li in both electrolytes questions a relevant impact of mass transport on its formation, at least in SPEs.