Abstract
While high sulfur loading has been pursued as a key parameter to build realistic high-energy lithium-sulfur batteries, less attention has been paid to the cathode porosity, which is much higher in sulfur/carbon composite cathodes than in traditional lithium-ion battery electrodes. For high-energy lithium-sulfur batteries, a dense electrode with low porosity is desired to minimize electrolyte intake, parasitic weight, and cost. Here we report the profound impact on the discharge polarization, reversible capacity, and cell cycling life of lithium-sulfur batteries by decreasing cathode porosities from 70 to 40%. According to the developed mechanism-based analytical model, we demonstrate that sulfur utilization is limited by the solubility of lithium-polysulfides and further conversion from lithium-polysulfides to Li2S is limited by the electronically accessible surface area of the carbon matrix. Finally, we predict an optimized cathode porosity to maximize the cell level volumetric energy density without sacrificing the sulfur utilization.
Highlights
While high sulfur loading has been pursued as a key parameter to build realistic high-energy lithium-sulfur batteries, less attention has been paid to the cathode porosity, which is much higher in sulfur/carbon composite cathodes than in traditional lithium-ion battery electrodes
Host materials have been incorporated with elemental sulfur to increase the electrical conductivity and sulfur utilization
The amount of electrolyte is critical for the volumetric energy density of batteries, since it accounts for a major part of the total cell weight[40]
Summary
While high sulfur loading has been pursued as a key parameter to build realistic high-energy lithium-sulfur batteries, less attention has been paid to the cathode porosity, which is much higher in sulfur/carbon composite cathodes than in traditional lithium-ion battery electrodes. The major issues facing its broader applications are: the intrinsic insulating characteristics of sulfur, the shuttle phenomenon that results from the high solubility of lithium polysulfide (Li-PS), the volume expansion of sulfur during lithiation, and the highly reactive Li-metal surface induced side reaction and mossy/dendrite growth[7,8,9] To address these issues, host materials have been incorporated with elemental sulfur to increase the electrical conductivity and sulfur utilization. If the porosity of the electrode is reduced by 10%, the gravimetric energy density may reach 500 Wh kg−1 due to the reduction of electrolyte quantity This is the target of the Department of Energy Battery 500 program for the advanced battery of electric vehicles[42]. The porosity design leads to an optimized design of Li–S battery and make it more attractive for the niche market
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