*2.2.2 Capping layer of MTO-CNTs for sulfur cathode*

The thin barrier layers designed with light weight and with high polysulfidetrapping capabilities showed high weight density (usually >0.3 mg cm−2), unexpectedly reducing the overall energy densities of Li-S batteries as shown in **Figure 3** [47–55]. The development of lightweight MTO-CNTs capping layer directly coated onto the surface of sulfide cathode as shown in **Figure 4a**. The MTO-CNTs capping layer prepared on the sulfur cathode, which is directly contacted with an electrically conducting layer to form a cathodic "sub cell" for capturing and decreasing the polysulfide species. The thickness (~2 μm) and the weight density (0.06 mg cm−2) of the MTO-CNTs capping layer are much lower than other barrier layers reported elsewhere, as the mesoporous carbon and the grapheme layers [47, 48]. Moreover, it is noted that the area of conventional interlayer is higher than the coated capping layer. To understand the layer-by-layer electrode structure and the cathode structure was observed by SEM and energy dispersive X-ray spectroscopy (EDX). The 1D, MTO-CNTs nanostructure and the ultrathin capping layer are formed through self-weaving and firmly coated on the surface of sulfur cathode (**Figure 4b**). The existence of MTO-CNTs capping layer is estimated by the elemental mapping analysis (**Figure 4c, d**). The MTO-CNTs capping layer formed on the cathode can trap more effectively, polysulfides within the cathode material, thereby reducing the shuttle effect of polysulfide. The CV curves of electrodes reveal that the electrode with MTO-CNTs capping layer has sharper and more intensive oxidation and reduction peaks exhibited than an electrode without capping layer, indicating that the capping layer efficiently enhanced reaction kinetics of the electrode. The substantially promoted charge transfer is further confirmed by EIS analysis. The sulfur cathode with MTO-CNTs capping layer presents much lower charge-transfer resistance compared with cathode without MTO-CNTs capping layer, indicating that

#### **Figure 3.**

*Schematic diagram of as fabricated Li-S battery using MTO-CNTs interlayer. Reprinted from ref. [41].*

*Advanced Chalcogen Cathode Materials for Lithium-Ion Batteries DOI: http://dx.doi.org/10.5772/intechopen.103042*

#### **Figure 4.**

*(a) The dripping of MTO-CNTs on the surface of sulfur electrode. (b) SEM images of the sulfur electrode with MTO-CNTs capping layer. (c) Corresponding Ti and (d) S elemental mapping. (e) Charge-discharge curves of sulfur electrode comprises MTO-CNTs capping layer at various C rates. (f) Cycling stability of the MTO-CNTs capping layer on sulfur electrode at 0.5 C. [reprinted from ref. [41].*

improvement of redox-conversion ability as well as conductivity. These results indicate that the capping layer is not only favorable for adsorption confinement of polysulfides within the electrode, but also increasing charge transfer, accelerating reversibility of polysulfide conversion. The galvanostatic charge-discharge profiles of the device with the capping layer were recorded at different rates as shown in **Figure 4e**. At the 0.2 C rate reversible capacity of 1212 mAh g−1 has been achieved. The specific capacities of 922 and 606 mAh g−1 were delivered at the high rates of 0.5 and 1 C, respectively. The cycling stability of the device with the capping layer at 0.5 C rate still it retains a capacity of 577 mAh g−1 after 500 cycles with capacity decay rate of 0.07% per cycle (**Figure 4f**), indicating a good cycling stability. These results indicate that the formation of MTO-CNTs capping layer is convenient route to fabricate high performance Li-S batteries with sulfur host along with commercial carbon materials.

The cell was disassembled after 100 cycles at 0.2 C, to understand the function of the MTO-CNTs capping layer at a potential of 2.8 V. The dimethyl carbonate solution was used to wash electrodes with capping layer and their structure analyzed by X-ray microtomography (XRM) and SEM. The overall structure and morphology of the MTO-CNTs are similar to that of the original sample as shown in **Figure 5a**, the SEM images of the capping layer after 100 cycles. The TiO2 volume cannot change due to its robustness and the interaction of the CNTs. The layer-by-layer stacked structure was indicated by a 3D reconstruction of the electrode (**Figure 5b**). The capping layer is uniformly and strongly anchored on the sulfur cathode surface and worked as a good absorbent layer to keep polysulfide species rather than to diffuse into the lithium anode. The signal of sulfur precipitate is very strong and uniformed in the cathode (**Figure 5c**), reveals that the polysulfide shuttle behavior retained by the MTO-CNTs

#### **Figure 5.**

*(a) SEM picture and (b) 3D XRM picture and (c, d) partial 2D of the sulfur electrode with MTO-CNTs capping layer at a potential of 2.8 V after 100 cycles at 0.2 C. reprinted from ref: [41].*

capping layer. The charge products are clearly seen in the capping layer execute the recycling of trapped polysulfides. **Figure 5d** reveals that the capping layer on surface of the sulfur cathode retain as it is without any cracks after cycling indicates that the structural stability. These results indicate that the light MTO-CNTs capping layer coated on the surface of sulfur cathode enhanced battery performances.
