Operando characterization of active surface area and passivation effects on sulfur-carbon composites for lithium-sulfur batteries

• First quantification of the passivation of the carbon matrix in Li-S batteries. • Electrochemically active specific surface area of carbon quantified by impedance. • Method of analysis validated with model carbon and carbon-sulfur composite electrodes. Sulfur electrodes for lithium-sulfur batteries necessarily contain a conductive additive, typically carbon, to enable the electrochemical reactions, since sulfur and the discharge product, Li 2 S, are insulators. Consequently, the full passivation of carbon, by deposition of sulfur and/or Li 2 S, would necessarily produce the death of the battery. However, here we demonstrate that for high-performance lithium-sulfur batteries operated under lean electrolyte conditions (electrolyte to sulfur ratio of 6 µL mg S −1 in Li-S coin cells), the extent of passivation of carbon is not severe enough to limit performance. This is shown by performing impedance measurements of fully charged lithium-sulfur batteries, from which we demonstrate that we can evaluate the specific surface area of carbon, and we find that the capacity fade with cycling is not due to a decrease in the electrochemically active surface area of carbon. These results show that introducing a higher surface area carbon in the sulfur electrode formulation is not needed to prevent passivation, and that the focus of lithium-sulfur development should be directed towards other issues, such as mitigating undesirable reactions at the lithium electrode and achieving robust sulfur electrode structures enabling fast transport of electrolyte species and, thus, more homogeneous reactions.