This theoretical study not only reveals time and space dependence of ion motion in the PEO/LiTFSI electrolyte having negative transference number, but also provide guidance to experimentalists seeking to detect the direction of ion motion in these electrolytes.
Significance and Impact
The framework allows us to examine the roles of diffusion and migration, and how their relative importance changes as the ion velocities and salt concentration profile in the cell evolve under the constraint of electroneutrality.
- Based on a concentrated solution theory, we develop relationships for the ion velocity of species in the PEO/LiTFSI electrolyte comprising separable contributions due to diffusion and migration.
- We simulated the behavior of lithium-PEO/LiTFSI-lithium symmetric cells with two different salt concentrations, one that has been shown to have positive transference number and the other with negative transference number.
- The model results show that impact of negative transference numbers was most evident at short times in regions far from the electrode surfaces; Li+ moves toward the positive electrode and most of the current is carried by the TFSI- ions.
- Motion of Li+ toward the positive electrode lasts for only 275 s; the ion velocity reverses sign at that point in time.