Moving Down Group 1: Analytical Challenges and Current Trends for Solid Polymer Electrolytes in Post‐Li Battery Applications

The solid polymer electrolyte (SPE) field has traditionally struggled with sufficiently high cation conduction at room
temperature. The reason for this is that ion transport is usually coupled to polymer chain mobility and as such is strongly
temperature dependent. The diversification of battery technologies from Li-ion into Na- and K-ion batteries has revived efforts
in the 80s and 90s to develop a broad conceptional understanding and to find overarching trends of the ion transport
properties of SPEs based on Group 1 elements, Li+, Na+, K+, Rb+ and Cs+ (i. e. beyond Li). This development brings its own
set of challenges, starting from additional constraints to measure heavier cations. With a clearer aim towards tangible
battery applications, particularly electrochemical stability and interphase formation, and together with a rising sensibility of
sustainability aspects, other parameters have gained more relevance and opened new vectors of research. The associated
scientific challenges and recent developments of Group 1 based SPEs shall be reviewed.