Understanding effects of conductive additives in lithium-sulfur batteries

Since 2009, lithium-sulfur (Li-S) batteries have been attracting much research interest, ascribed to their high energy density and cost-effectiveness. However, their S cathodes are harassed by the poor conductivity, shuttling of lithium polysulfides (LPSs), and large volume change. In improving conductivity of S cathodes, carbon materials are widely used as conductive additives. However, there still lacks a comparative study on their geometric and structural effects in Li-S batteries. To this end, a systematic investigation is conducted on two different typical conductive additives, carbon black (CB)(1) and nitrogen-doped graphene nanosheets (N-GNS)(2). This work reveals that, compared to the zero-dimensional (OD) CB nanoparticles, the two-dimensional (2D) N-GNS is structurally less defective and has a larger surface area. As a result, the 2D structure of the N-GNS helps achieve higher capacity retention but inhibits Li-ion transportation. In comparison, the OD structure of the CB facilitates the transportation of Li-ions with a higher initial capacity but exposed to a severer shuttling behavior of LPSs. It is also found that a combination of the CB and N-GNS enables better performance of Li-S batteries, in terms of sustainable capacity, Coulombic efficiency, and rate capability. This study is inspiring for designing better S cathodes for Li-S batteries.