Shining Light on New Additives for Lithium Sulfur Batteries through Polysulfide Shuttle Tracking Operando Optical Fluorescence Microscopy
Authors: Coke Kofi , Soni Roby , Shearing Paul Robert , Miller Thomas Samuel 
Published: December 2024

Lithium Sulfur (Li-S) batteries are uniquely poised to provide exciting new opportunities in the field of energy storage, possessing an impressive theoretical gravimetric specific capacity an order of magnitude greater than current lithium-ion batteries (1672 mA h g-1), and energy density over 5 times higher (2567 Wh kg-1), while also using far safer and more sustainable materials.

However, their wilder proliferation and commercialisation suffers heavily from the polysulfide shuttle effect, where the lithium polysulfide intermediates generated at the cathode during the conversion mechanism of Li-S operation dissolve into the battery electrolyte and diffuse towards the lithium metal anode. This leads to parasitic reactions between the polysulfides and the anode, causing rapid cell degradation from active material loss, degradation of the anode solid electrolyte interface (SEI), and subsequent rapid dendrite formation. The shuttle effect is a primary mechanism of cell failure, and as such, most research into Li-S focuses on its mitigation. Current methods of characterizing the shuttle effect provide limited detail or are overly time consuming, so more robust characterization methods are required to further understand the distribution, dynamics, and degree of polysulfide transport.

Here, the newly established Optical Fluorescence Microscopy (OFM) technique, for visualising the spatial distribution of polysulfides within the Li-S electrolyte and through sufficiently large cathode pores during operando cycling, is utilised for facile assessment of the efficacy of a variety of additives to Li-S batteries in reducing the polysulfide shuttle effect. Using a polysulfide sensitive fluorescent dye, established to be inert to both the components of a Li-S battery and the additives, the intermediate polysulfides formed throughout cycling can be made to fluoresce, and thus directly visualised. The correlation between fluorescence intensity and polysulfide concentration is also linear when below the linear concentration range of the dye, allowing for polysulfide concentration to be spatially quantified through use of a calibration curve. This enables direct observation of the kinetics of sulfur dissolution and the subsequent diffusion towards the anode and dendrite formation, and thus measurement of the efficacy of steps taken to prevent this.

The effects of various additives are elucidated using this method – including the standard LiNO3 additive in sacrificially protecting the lithium anode through SEI formation, as well as novel fibroin-based additives trapping polysulfides within the cathode. Further, the total lack of solubilised intermediate polysulfides within the electrolyte potentially granted by a quasi-solid-state Li-S morphology is assessed through OFM. As is the impact of catalytic additives on the kinetics of the Li-S conversion mechanism, through reduction in the time polysulfides spend in the solubilised state. With OFM, one can directly and conveniently assess the real-time spatial distribution, dynamics, and degree of polysulfide transport in a cycling Li-S cell far faster and significantly easier then would be possible with other methods, and with a very robust dataset. OFM is thus an ideal platform for the development of polysulfide shuttle mitigation strategies, allowing rapid iteration of mitigation methods such as new additives or cathode morphologies. It has the potential for a widespread impact in the field of Li-S batteries, in both industry and academia, through collaboration with and usage by other researchers developing shuttle mitigation techniques.

Figure 1