Impact of Nano‐sized Inorganic Fillers on PEO‐based Electrolytes for Potassium Batteries

The low melting points of solid polymer electrolytes (SPEs) based on the KTFSI electrolyte salt allow comparatively low operation temperatures (below 50 °C) for K-ion batteries, unlike their Li or Na counterparts. Unfortunately, for this reason the electrolyte is also rendered mechanically unsuitable in its function to act as a cell separator. Therefore, in this work the use of inorganic nanofillers (Al$_2$O$_3$ and SiO$_2$) is explored with the aim to improve rheological, thermal and cation transport properties of the resulting polymer composite electrolytes. Their electrochemical properties were further examined in K-metal symmetrical cells and K-metal/SPE/K$_2$Fe[Fe(CN)$_6$] cells and compared to corresponding liquid electrolyte systems. As a result of particle-polymer interactions, filler-containing SPEs showed higher degrees of crystallinity combined with filler polymer interaction and thus improved mechanical integrity in the relevant temperature range of 25–55 °C, while maintaining similar ionic conductivities than a filler-free sample above the melting temperature. Although plating-stripping experiments in symmetrical cell setups suggested high cell resistances for various compositions and in some cases even rapid cell failure, Al$_2$O$_3$-based SPEs generally displayed high capacity retention when cycled against a positive electrode (here Prussian blue analogue K$_2$Fe[Fe(CN)$_6$]) over 100–160 cycles and possibly beyond.