Multivalent Cation Transport in Polymer Electrolytes : Reflections on an Old Problem

Today an unprecedented diversification is witnessed in battery technologies towards so-called post-Li batteries, which include both other monovalent (Na+ or K+) and multivalent ions (e.g., Mg2+ or Ca2+). This development is driven, among other factors, by goals to establish more sustainable and cheaper raw material platforms, using more abundant raw material, while maintaining high energy densities. For these new technologies a decisive role falls to the electrolyte, that ultimately needs to form stable electrode-electrolyte interfaces and provide sufficient ionic conductivity, while guaranteeing high safety. The transport of metal-ions in a polymer matrix is studied extensively as solid electrolytes for battery applications, particularly for Li-ion batteries and are now also considered for multivalent systems. This poses a great challenge as ion transport in the solid becomes increasingly difficult for multivalent ions. Interestingly, this topic is a subject of interest for many years in the 80s and 90s and many of the problems then are still causing issues today. Owing to recent progress in this field new possibilities arise for multivalent ion transport in solid polymer electrolytes. For this reason, in this perspective a stroll down memory lane is taken, discuss current advancements and dare a peek into the future.