Severe Loss of Confined Sulfur in Nanoporous Carbon for Li–S Batteries under Wetting Conditions

One key aspect in the cathode chemistry of lithium–sulfur (Li–S) batteries is the firm contact between elemental sulfur and the conducting carbon host to ensure the occurrence of the reduction reaction. In this respect, we study the retention of sulfur in the model cathode material CMK-8 carbon loaded by 20 and 50 wt % in the presence of solvents of different nature (D2O, d-THF, d-toluene) using small-angle neutron scattering. Exploiting the contrast matching between carbon and these liquids, the scattering of the sulfur phase is isolated and the amount and distribution of sulfur in the pores is determined in precise and direct fashion. The chord-length distribution (CLD) analysis shows that sulfur is preserved in both meso- and micropores if D2O is used. In the case of d-THF, sulfur is found in the micropores only, whereas complete removal from the pore network occurs for wetting with d-toluene. This severe loss in the presence of solvents similar to those used in Li–S batteries (e.g., dioxolane, diglyme) is ascribed to their superior adhesion on the carbon surface and, more importantly, indicates that only a little amount of loaded sulfur is available for the electrochemical reactions