Li-Ion Transport and Solvation of a Li Salt of Weakly Coordinating Polyanions in Ethylene Carbonate/Dimethyl Carbonate Mixtures

Abstract

Electrolytes with a high Li-ion transference number (<i>t</i>_Li) have attracted significant attention for the improvement of the rapid charge-discharge performance of Li-ion batteries (LIBs). Nonaqueous polyelectrolyte solutions exhibit high <i>t</i>_Li upon immobilization of the anion on a polymer backbone. However, the transport properties and Li-ion solvation in these media are not fully understood. Here, we investigated the Li salt of a weakly coordinating polyanion, poly[(4-styrenesulfonyl)(trifluoromethanesulfonyl)amide] (poly(LiSTFSA)), in various ethylene carbonate and dimethyl carbonate mixtures. The highest ionic conductivity was unexpectedly observed for the lowest polar mixture at the highest salt concentration despite the low dissociation degree of poly(LiSTFSA). This was attributed to a unique conduction phenomenon resulting from the faster diffusion of transiently solvated Li ions along the interconnected aggregates of polyanion chains. A Li/LiFePO₄ cell using such an electrolyte demonstrated improved rate capability. These results provide insights into a design strategy of nonaqueous liquid electrolytes for LIBs.