material costs of cobalt (Co) and nickel (Ni) for LiNiMnCoO 2 (NMC) are 28.46 USD per lb and 5.57 USD per lb, respectively. [8] In order to secure cost effectiveness, lithium-sulfur (Li-S) batteries have been considered one of the most attractive energy storage systems because of their high theoretical capacity (1672 mAh g −1 ), energy density (2600 Wh kg −1 ), nontoxicity, low cost, and the natural abundance of sulfur. [9,10] Nevertheless, there are obstacles that should be overcome for the development of Li-S batteries, including the insulating nature of sulfur (5 × 10 −30 S cm −1 ), the large volume change of sulfur (≈80%), and the poor cycle performance by the shuttle phenomenon. In particular, the shuttle phenomenon is caused by the high solubility of the lithium polysulfide intermediate (Li 2 S X , 2 < X ≤ 8) in the electrolyte. These lithium polysulfides diffuse to the Li metal anode, leading to a loss of sulfur from the sulfur electrode and low coulombic efficiency (CE). [11,12] To resolve these issues, most studies have focused on fabricating an inhibitor to trap the lithium polysulfides in the cathode side through chemical/physical adsorption, such as the interlayer inserted between the sulfur electrode and the separator with a metal oxide, [13][14][15] conductive carbon material, [16] polymers, [17][18][19][20][21][22] Al 2 O 3 , [23][24][25][26] Ni foam, [27,28] CoS 2 , [29,30] and metal carbide. [31,32] Among them, conductive carbon materials such as carbon fiber, [33][34][35][36][37] porous carbon, [38][39][40][41][42] graphite, [43][44][45] graphene Developing a highly effective interlayer inserted between the sulfur electrode and separator is one of the most important issues in Li-S battery research, because this interlayer enhances the cycle performance of the Li-S battery by trapping the lithium polysulfides in the sulfur electrode. Among various interlayer materials, carbon materials such as graphene and carbon nanotubes are particularly appealing because of their high electrical conductivity.
Here, a new flexible carbon membrane interlayer consisting of graphene oxide (GO) and carbon nanotubes (CNTs) is developed by a facile vacuum filtration approach to trap the lithium polysulfides in the sulfur electrode.When the GO/CNT bilayer membrane is used as an interlayer between the sulfur electrode and separator (glass fiber), the Li-S battery delivers an initial discharge capacity of 1591.56 mAh g −1 and maintains a capacity of about 1000 mAh g −1 over 50 cycles at 0.2C with a low potential difference of 150 mV. This reflects higher electrochemical performance than a GO or CNT monolayer unilaterally. This is attributed to the hydrophilic functional group of the GO layer strongly adsorbing lithium polysulfides dissolved in liquid electrolyte and the CNT layer enhancing the ion conductivity.
