Overlimiting ion transport dynamic toward Sand's time in solid polymer electrolytes

The ion transport dynamics in solid polymer electrolytes can be well captured by the classic Nernst-Planck equation under the dilute solution assumption. However, investigations on the characteristic Sand's time under various overlimiting currents are quite limited, partly due to the difficulties of determining the true current density and the accurate starting time of dendritic growths. Here, transparent microcapillary cells are fabricated to overcome these challenges. Our specialty cells not only minimize the possible discrepancy between the geometric current density and the true local current density by reducing the cross-sectional area, but also allow reliable determination of the Sand's time via direct operando optical observation. Sand's time simulations using both the dilute solution theory and the concentrated solution theory, with the parameters cross-validated by independent measurements, match closely with the experimental Sand's time. Our work demonstrates that the onset of lithium dendritic growths in solid polymer electrolytes may not always yield the familiar Sand's time voltage spike. Avoiding the localized overlimiting current density is the key to developing penetration-free polymer electrolytes.