Advanced Lithium-Sulfur Batteries with Carbon Nanotubes As Conductive Networks

Lithium-sulfur batteries, with the high theoretical capacity (1675 mAh g^-1) and energy density (2600 Wh kg^-1) as well as the easy access and non-toxic of raw materials, demonstrate a promising prospect among the high energy storage systems for electric vehicles (EVs) and portable electronic devices. However, there are also several issues to be handled before commercial application.

Firstly, different from traditional lithium-ion batteries, intermediate polysulfides generate and dissolve in the organic electrolyte during the discharge process of lithium-sulfur batteries. The dissolved polysulfides, shuttle between cathode and anode sides, lowering the utilization of sulfur and also react with the lithium anode without any electron transfer. However, the dissolution of polysulfide in a lithium-sulfur battery is highly dependent on the sulfur/electrolyte loading. Thus an appropriate dosage of electrolyte will build a concentration gradient, which will limit the dissolution of polysfulfides. In our work, A lithium-sulfur cell with a high initial discharge capacity of 1053 mAh g^-1 at a high rate of 1 C and an ultralow decay rate of 0.049 %/per cycle during 1000 cycles was obtained by using CNT-sulfur cathode and suppressing polysulfide shuttle to a shuttle factor of 0.02 by matching the sulfur/electrolyte loading. Secondly, sulfur is with low electrical conductivity (5×10^-30 S cm^-1 at 25 ^oC) and extra conductive network is highly required. Generally, the addition of conductive materials will neutralize the advantage in high energy density of lithium-sulfur battery. Thus, the more sulfur content in the composite cathode, the larger energy density. Herein, we employed the carbon nanotubes (CNTs) to build the unblocked conductive skeleton and a room-temperature, one-step ball-milling treatment of aligned CNTs and sulfur was applied to obtain high sulfur content of 90 %. The 90 % sulfur loading cathode exhibited a superior density of 1.98 g cm^-3 (2.07 g cm^-3for sulfur), which will greatly increase the volumetric energy density of lithium-sulfur battery.

Reference:

[1] XB Cheng, et al. J. Power Sources, 2014, DOI: 10.1016/j.jpowsour.2013.12.031

[2] XB Cheng, et al. Nano Energy, 2014, accepted