How do organic polysulphides improve the performance of Li-S batteries?

Lithium-sulphur (Li–S) batteries are promising energy storage systems, but it suffers from several limitations mainly due to the electrolytes. In this study, we explore the effect of organic sulphides on the modification of the system performance of Li–S. We compare in this study, the effect of catholytes based on three differently functionalized organic sulphides as models. These include an activated thiol (tri-fluoro methane benzene thiol: BTFBT) (1), a weakly activated aromatic disulphide (diphenyl disulphide: Ph2S2) (2) and a di-thiol deactivated by a di-ether function: DMDO) (3). The galvanostatic discharge profile of sulphur based-cathode exhibits an additional first plateau corresponding to the electrochemical redox voltage of the disulphide (or thiol) initially present in the catholyte. Their reduction voltage (vs. Li+/Li) follows the order BTFBT > S8 ≈ Ph2S2 > DMDO and is directly dependent on the electron withdrawing/donating nature of the organic sulphide. The presence of the organic sulphide in the catholyte increases the capacity utilization of mineral sulphur of the cathode by approximately 33.8%, 49.3% and 27.9% in case of BTFBT, DMDO and Ph2S2 respectively at a C-rate of C/5. To explain these observations, we propose potential chemical balance between the redox systems (S8/Sx2−) and (RSxR/RSx−) in the solution kinetically and thermodynamically controlled. Furthermore, the lithium anode behaviour was also probed by lithium stripping/deposition measurements. Results indicate that the presence of 0.2 M Ph2S2 in the catholyte increases the overvoltage by ∼32 mV due to deposited of stable protective lithium thiolate layer as observed by SEM. Finally, long term cycling of 500 cycles validates the positive effect of the organic catholytes as chemical barrier again sulphur loss during cycling.