Lithium-sulfur (Li-S) batteries hold the promise of the next generation energy storage system beyond state-of-the-art lithium-ion batteries. Despite the attractive gravimetric energy density (WG ), the volumetric energy density (WV ) still remains a great challenge for the practical application, based on the primary requirement of Small and Light for Li-S batteries. This review highlights the importance of cathode density, sulfur content, electroactivity in achieving high energy densities. In the first part, key factors are analyzed in a model on negative/positive ratio, cathode design, and electrolyte/sulfur ratio, orientated toward energy densities of 700 Wh L-1 /500 Wh kg-1 . Subsequently, recent progresses on enhancing WV for coin/pouch cells are reviewed primarily on cathode. Especially, the "Three High One Low" (THOL) (high sulfur fraction, high sulfur loading, high density host, and low electrolyte quantity) is proposed as a feasible strategy for achieving high WV , taking high WG into consideration simultaneously. Meanwhile, host materials with desired catalytic activity should be paid more attention for fabricating high performance cathode. In the last part, key engineering technologies on manipulating the cathode porosity/density are discussed, including calendering and dry electrode coating. Finally, a future outlook is provided for enhancing both WV and WG of the Li-S batteries.
Keywords: electrocatalytic activity; host materials; lithium-sulfur batteries; sulfur cathodes; volumetric energy density.
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