Effect of specifically-adsorbed polysulfides on the electron transfer kinetics of sodium metal anodes

Abstract

Room-temperature sodium-sulfur (RT Na-S) batteries hold great promise for large-scale energy storage applications owing to the high energy density and earth-abundance of Na and S. However, the dissolution and migration of sodium polysulfides, uncontrollable Na dendrite growth, and the lack of studies on Na electrodeposition kinetics have hindered the development of these batteries. Herein, we reveal the mechanism of sodium polysulfides on the Na plating/stripping kinetics using a three-electrode system. First, the kinetic behavior deviates from the commonly supposed Butler–Volmer model, which is well described by the Marcus model. In addition, the specific adsorption of polysulfides on the sodium electrode surface is a key factor influencing the kinetics. Higher-order polysulfides (S82− and S62−) exhibit distinct specific adsorption behaviors because of their high adsorption energies compared to lower-order polysulfides (S42− and S22−). The electrostatic effect caused by specific adsorption can accelerate the kinetics, whereas the blocking effect can slow the kinetics. Thus, this competitive relationship enables low concentrations of high-order polysulfides to stimulate kinetics. This implies that a weak shuttle effect is beneficial for obtaining a stable Na deposition in RT Na-S batteries. An in-depth understanding of the Na electrodeposition kinetics provides beneficial clues for future metal sodium/electrolyte interface designs.