Electrochemical redox kinetic behavior of S8 and Na2Sn (n = 2, 4, 6, 8) on vulcan XC-72R carbon in a flowing-electrolyte system

Abstract

Many studies have shown that sulfur electrodes in room-temperature sodium–sulfur (RT Na–S) batteries undergo a liquid-phase electrochemical reaction. Moreover, the electrochemical redox kinetic behavior of S8 and sodium polysulfides (Na2S8, Na2S6, Na2S4, and Na2S2) is important in understanding the macroscopic kinetics of sulfur electrodes. In this study, a three-electrode electrochemical test system is built using a two-chamber micro-electrolysis cell with a flowing electrolyte and Nafion membrane. Electrochemical kinetic parameters, such as the onset redox potentials, exchange current densities, reaction orders, and apparent activation energies of the sodium polysulfides during the redox process are obtained from a steady-state polarization curve. The onset redox potentials of S8, Na2S8, Na2S6, Na2S4, and Na2S2 on Vulcan XC-72R carbon are 2.37, 2.23, 2.11, 1.99, and 1.83 V (vs. Na/Na+), respectively. Furthermore, the relationship between the discharge capacity of sodium polysulfides and their onset redox potentials is established. The aforementioned kinetic parameters are vital in elucidating the formation of discharge/charge platforms and sloping regions in an RT Na–S battery. The results of this work provide a basis to further understand the performance of RT Na–S batteries.