Lithium Sulfonate Functionalization of Carbon Cathodes as a Substitute for Lithium Nitrate in the Electrolyte of Lithium-Sulfur Batteries

A method for grafting lithium sulfonate (LiSO3) groups to carbon surfaces is developed and the resulting carbons are evaluated for their potential to reduce the lithium polysulfide (LiPS) shuttle in lithium-sulfur (Li-S) batteries, replacing the common electrolyte additive lithium nitrate (LiNO3). The LiSO(3)groups are attached to the ordered mesoporous carbon (CMK3) surface via a three-step procedure to synthesize LiSO3-CMK3 by bromomethylation, sodium sulfite (Na2SO3) substitution, and cation exchange. As a comparison, ethylenediamine (EN)-substituted CMK3, EN-CMK3, is also synthesized and tested. When used as a cathode in Li-S batteries, the unfunctionalized CMK3 suffers from strong LiPS shuttling as evidenced by its low initial Coulombic efficiencies (ICEs, <10%) compared to its functionalized derivatives EN-CMK3 and LiSO3-CMK3 (ICEs >75%). Postcycling analysis reveals the benefits of cathode surface functionalization on the lithium anode via an attenuated LiPS shuttle. When monitored at open circuit, the functionalized cathodes maintain their cell voltages much better than the CMK3 control and concurrent electrochemical impedance spectroscopy reveals their higher total cell resistance, which provides evidence for a reduced LiPS shuttle in the vicinity of both electrodes. Overall, such surface groups show promise as cathode-immobilized lithium nitrate mimics.