High-energy-density Li–S battery with positive electrode of lithium polysulfides held by carbon nanotube sponge

Lithium-sulfur battery suffers from the low utilization of sulfur and the high electrolyte/sulfur (E/S) ratio that decrease the cell-based performance. Lithium polysulfides (Li₂S_x)-dissolved electrolyte, so called catholyte, enables high utilization of sulfur, but the cell inherently has high E/S ratio due to the limited solubility of Li₂S_x. Herein, a composite electrode of Li₂S_x (x = 4, 6, 8) and sub-millimeter-long few-wall carbon nanotube (CNT) is proposed. The CNT forms self-supporting sponge-like paper and works as a three-dimensional current collector, in which the Li₂S_x is deposited by solution casting and drying. The Li₂S₆-CNT electrode realizes high specific capacity (1249 mAh g_sulfur⁻¹) under a lean electrolyte condition of E/S = 4 μL mg_sulfur⁻¹, which is much better than the S₈-CNT electrode (233 mAh g_sulfur⁻¹). After full charge and conversion of Li₂S_x to S, the Li₂S_x-CNT electrode maintains its high capacity of 1100 mAh g_sulfur⁻¹. The full cell with the Li₂S₆-CNT and the Li thin foil electrodes realizes 400–500 Wh kg_cell⁻¹ for E/S = 4.0 at the 2nd and 3rd discharge and 300 Wh kg_cell⁻¹ for E/S = 5.8 at the 97th discharge, based on the total mass of the interior of a cell (electrodes, separator, and electrolyte). Holding solvated Li₂S_x by the CNT sponge is the key for the high energy density.