Cycle-Induced Structural Evolution of Sulfur Cathodes in Lithium-Sulfur Batteries
Authors: Manjum Marjanul , Lateef Saheed Adewale , Mustain William Earl , Jalilvand Golareh 
Published: November 2022

Lithium (Li) ion batteries (LIBs) have been the predominant and fastest growing energy storage technology over the past few decades. A significant amount of LIB research has been carried out and remarkable improvements in the technology have been achieved. As a result, state-of-the-art LIBs offer superior cyclability, high efficiency, and high specific energy relative to competitors [1]. However, the desire for long-range electric vehicles (EVs) and grid-level energy storage and delivery is increasing the demands for batteries with very high gravimetric energy density (e.g. > 500 Wh/kg) [2]. This is simply much higher than what LIB electrode materials can practically offer (~ 260 Wh/kg). Therefore, alternative chemistries are needed at both electrodes.
One material that has received significant attention recently as a replacement cathode material in Li-based batteries is sulfur (S). S has 5 times the theoretical specific energy than conventional LIB cathodes and can offer a practical energy density of > 500 Wh/kg when coupled with commercially available lithiated graphite or Li metal anodes [3]. S is also non-toxic, low-cost, and has high natural abundance. These properties make S a promising candidate for next-generation cathodes in Li battery systems. Yet, the path to achieving near theoretical capacity and long cycle life for S cathodes has proven difficult due to numerous unsolved scientific and technical issues including: i) the insulating nature of Sulfur (S8) and its discharged product (Li2S); ii) undesired solubility of the S products in the liquid electrolyte, resulting in the degrading so-called Li polysulfides “shuttling”, and iii) structural change of the S cathode during charge and discharge due to the large volume variation between the fully charged and discharged products [4].
Several approaches have been reported to address these challenges and improve the Li-S battery performance and durability. Despite these efforts, the advances have been mostly limited to a small number of cycles, or the need for complex structures and that would lead to expensive synthesis costs at the manufacturing scale. In fact, it is not truly known if such complex structures are even necessary as the literature lacks a truly systematic investigation into i) the influence of the S structure on its behavior; and ii) how the S structure evolves as a result of charging and discharging the cell. It is also likely that complex structures would not be reformed upon deep charging/discharging – making their possible advantages only temporary. Hence, there are a limited number of truly practical S cathodes that can be rationally developed [3, 4].
In this work, new insights are presented regarding the structural evolution of S cathodes throughout cycling. The structural changes experienced by the S cathodes were investigated by scanning electron microscopy (SEM) during charge and discharge (at C/10) over the lifetime of the cell (10’s to 100’s of cycles) for multiple cells. Cycling was done with Li-S coin cells that were made using a Li metal anode and a S cathode. The S cathode was prepared using commercially available S powder, a through low-cost, simple, and scalable electrode recipe and production techniques. Drastic microstructural and compositional transformations were observed in the S cathodes as a consequence of charging and discharging. Results suggest that there was a reversible swelling transfiguration of the support structure (conductive carbon plus binder) during each discharge and charge step. It was also observed that the location and distribution of S was changed, and new structures were formed. These results are expected to cast light on a fairly unknown area in the Li-S battery technology, which can help with future scale-up and manufacturing of these cells.

References

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