Study of Morphological Changes in the Cathode Electrodes of Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) battery is a promising candidate for the next-generation batteries for electric vehicles due to its high theoretical specific energy and the low cost of sulfur. However, the challenges in the battery performance like low volumetric energy density, poor cycle life, and high self-discharge rate are yet to be resolved. To achieve the better understanding on the root causes behind the numerous technical challenges and to provide the right directions to solve the problems, the development of characterization techniques to understand the reaction mechanisms and the phenomena inside the Li-S cells are highly required. In this study, the morphological changes of S-LVO (LiV₃O₈) composite were analysed to investigate the reason why S-LVO composite shows better battery performance over the pristine sulfur material [1].

Experimental

S-LVO composite was manufactured by a mechano-fusion process using a Nobilta equipment to form the LVO coating on sulfur particles as shown in Figure 1.To analyse the cathode electrode, lithium // Celgard 3501 / LiTFSI 1M in DME/DOL (1:1) // sulfur coin cells were prepared and  opened at various values of death of discharge (DOD) in the first cycle. The cathodes were rinsed and dried (at 0.65 atm, 55°C  for 1h), then observed in the scanning electron microscope (SEM) using a specially designed transfer chamber to avoid any air/moisture contact between the SEM and the glove box. Secondary electrons (SE) images, backscattered electron (BSE) images and comparative X-ray elemental analysis using an energy dispersive spectrometer (EDS) were performed at 15 keV.

Results and discussion

Figure 2 shows the discharge profile in the 3^rd cycle at 0.1C condition and the cycle performance at 0.5C condition in comparison between the S-LVO composite electrode and the reference sulfur electrode. It can be seen that the S-LVO outperforms the pristine sulfur in the initial capacity and the cycle life. Figure 3(a) demonstrates that, as the DOD increases from 0 to 100%, the  sulfur-based cathode undergoes a significant densification and tend to form a severe “mud-crack” morphology as shown in Figure 3(a), while this effect is relatively smaller on the S-LVO composite cathode (Figure 3(b)).  This densification process is well seen in the SE image in Figure 3(c). It is believed that the dispersed LVO particles retard the densification of electrode and contribute to preserve the mechanical integrity of electrode. Figure 4 shows the evolution of sulfur-to-carbon ratio (S/C) that was measured at constant values of electron beam current, working distance and collection. It is found that the S-LVO electrode shows a lower variation of the S/C ratio, which indicates that S-LVO electrode has relatively homogeneous sulfur distribution while the concentration of sulfur is more localized on the electrode surface in the case of the reference electrode.

Reference

[1] C-S Kim et al, ‘Facile dry synthesis of sulfur-LiFePO4 core-shell composite for scalable fabrication of lithium/sulfur batteries’, Electroch. Comm, 32 (2013) 35-38.