Competing effects of low salt ratio on electrochemical performance and compressive modulus of PEO-LiTFSI/LLZTO composite electrolytes†

Polyethylene oxide (PEO)-based solid composite electrolytes (SCEs), with inorganic fillers, are studied extensively due to their effective balance between mechanical and electrochemical properties. The correlation between the composition of SCEs and their electrochemical behavior has been studied extensively, primarily focusing on the type of polymer matrix with a bias towards high lithium (Li) salt. In this study, we examine the changes in the properties of SCEs at two low EO : Li ratios, 43 : 1 and 18 : 1, in the PEO-LiTFSI matrix (with and without 10 wt% of 5 μm LLZTO) and evaluate their impact on Li stripping and plating reactions. Although higher salt concentration (18 : 1) results in substantially higher ionic conductivity (by approximately an order of magnitude), interestingly we observe that lower salt concentration (43 : 1) exhibits up to 3 times longer Li cycling life. Notably, electrolytes with low salt concentration (43 : 1) are much stiffer, with compressive modulus more than twice as high as the 18 : 1 counterpart. Although the ionic conductivity of the electrolyte is often the most immediate concern in the electrolyte design process, these findings accentuate the equal importance of mechanical properties in order to ensure successful electrolyte performance throughout prolonged Li cycling.