Investigation of Lithium-Sulfur Battery Performance with Free-Standing Amide-MWCNT Protection Layer
Authors: Lai Yong-Hong , Chen Hsun-Yi 
Published: February 2020

Lithium-Sulfur (Li-S) batteries are considered as next generation energy storage for its low-cost, abundant and environmental-friendly cathode material and high theoretical energy density (2,600 Wh/kg), which is over five-fold of the state-of-art lithium-ion batteries. This promising battery technology still faces some challenges, and the most infamous one is the “shuttle effect” induced by high-order polysulfide intermediates, which leads to fast degradation of the battery. Significant efforts have been devoted to solve this problem for years and efficient methods for suppressing the shuttle effect can be categorized into four kinds: confining, trapping, blocking and breaking up. Blocking is by employing an interlayer between the separator and the cathode to limit the migration of high-order polysulfide. The interlayer made of multiwall carbon nanotubes (MWCNT) have shown great ability to limiting the diffusion of polysulfide, but the nonpolar carbon surface only weakly interacts with polar polysulfide.
In order to increase the interaction with the polar polysulfide, modifying the carbon surface is necessary by doping heteroatom, modification with the polar compound and inducing functional groups. In this work, we equip MWCNT with amide group by reacting it with ethylenediamine (EDA) to increase its trapping ability to polysulfide, which can in turn further improve the efficiency of carbon interlayer in limiting the migration of polysulfide. Then, the amide-MWCNT (A-MWCNT) is mixed with the pure MWCNT to fabricate the free-standing composite interlayer. The characteristics of A-MWCNT composite interlayer was measured by X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FTIR) to confirm the functionalization process. The trapping ability of the A-MWCNT composite interlayer was also investigated in a testing platform of Li-S batteries assembled with the composite interlayer. Cyclic voltammetry (CV) and galvanostatic charge/discharge experiments are conducted to examine undesired side reactions and determine its electrochemical reversibility.
XPS analysis results are shown in figure 1. (a). there is an additional signal from N 1s signal(400 eV) after functionalization, providing evidence showing that the MWCNT surface is equipped with the amide groups. Electrochemical stability and reversibility of the Li-S batteries with A-MWCNT composite interlayer are demonstrated in figure 1. (b). The cycle life of the batteries with A-MWCNT composite interlayer can reach 200 cycles and high specific capacity is still sustained. We also compare the performance of Li-S batteries with and without functionalized MWCNT interlayers, and the former showed lower decay rate and higher capacity retention. The rate capability and cyclability of Li-S batteries with A-MWCNT composite interlayer will be detailed reported. Our work demonstrates that a multi-functional interlayer with trapping and blocking abilities can suppress the shuttle effect and bring Li-S batteries closer to commercialization.

Figure 1