Volumetric discharge capacity 1 a h cm ^-3 realized by sulfur in carbon nanotube sponge cathodes

To make lithium-sulfur batteries competitive with commercialized lithium-ion batteries, sulfur cathodes need to be of high sulfur content, thick, and dense. We use 35 μm-thick, spongelike free-standing papers of carbon nanotubes (CNTs) as three-dimensional conductive matrices and simply capture sulfur in them by sublimation without using any binder. Because of good wettability of sulfur on the CNT surface and careful control of the temperature difference between the sulfur source and the CNT paper, sulfur is deposited uniformly on the CNT paper at controllable contents of 20-80 mass %. The 23 μm-thick, 1.5 g cm ^-3 -dense electrode with 70 mass % sulfur showed a breakthrough volumetric discharge capacity of 1100 mA h cm ^-3 with good gravimetric and areal discharge capacities of 1100 mA h g _sulfur ^-1 , 763 mA h g _electrode ^-1 , and 2.67 mA h cm ^-2 at the eighth cycle. Furthermore, the electrode retained the very high discharge capacity of 990 mA h cm ^-3 after 100 cycles. Such high performance is realized by employing our submillimeter-long, 99 mass %-pure, few-wall CNTs with careful control over the sulfur deposition process. Simple is the best; sulfur in the CNT sponge architecture minimizes additives and maximizes sulfur content and is promising to realize sulfur cathodes with practically high values for any of gravimetric, areal, and volumetric capacities.