Impact of Composition and Viscosity on Ion Transport and Dendrite Resistance in Viscoelastic Polymer Electrolytes

This study investigates the ion transport, rheological, and electrochemical properties of a series of poly(vinyl ethylene carbonate - lithium styrene bis(trifluoromethanesulfonyl)imide) (p(VEC-LiSTFSI)) polymer electrolytes. Three different molar ratios of VEC to LiSTFSI are synthesized (5:1, 10:1, and 20:1), resulting in materials with consistency ranging from viscoelastic liquids to low-viscosity liquids, a 10000-fold difference in zero-shear viscosity. The ionic conductivity at 40 °C increases with decreasing LiSTFSI content, from 1.88 to 5.49 x 10^-4 S/cm for the 5:1 and 20:1 ratios, respectively. However, this increase is accompanied by a significant drop in the Li⁺ transference number. Li symmetric cell cycling at 40 °C demonstrates that the polymer with the intermediate VEC to LiSTFSI ratio, 10:1, exhibits the best cyclability, rate capability, and dendrite resistance, lasting 411 hours at 0.2 mA/cm². Conversely, the lowest viscosity polymer (20:1 ratio), shows immediate shorting. These results highlight a critical trade-off where a moderate increase in zero shear viscosity, despite slightly lower conductivity, is crucial for significantly enhancing dendrite resistance and overall cell performance in these non-crosslinked electrolytes.