Development and characterisation of lithium sulfur batteries with operando optical fluorescence microscopy

As the world seeks to embrace more sustainable energy generation methods in the face of the climate crisis, Lithium Sulfur (Li-S) batteries are uniquely poised to provide exciting new opportunities in the field of energy storage. They possess 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). However, the wider proliferation and commercialisation of Li-S batteries is hindered significantly by the polysulfide shuttle effect, where lithium polysulfide (PS) charge carriers produced in the cathode during cycling diffuse through the electrolyte towards the lithium metal anode and engage in parasitic reactions. This leads to severe active material loss and degradation and serves as a primary mechanism of cell failure. A majority of work into Li-S focuses on the mitigation of the shuttle effect – so great is its detriment to Li-S battery performance – however characterisation of the efficacy of these methods proves consistently difficult, due to the time-consuming nature and limited detail provided by existing techniques. Here, the newly established Optical Fluorescence Microscopy (OFM) technique, for qualitatively visualising the spatial distribution of polysulfides within the Li-S electrolyte and 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 detected by fluorescence, 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 reduction 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 to within the cathode. Further, the total lack of solubilised intermediate polysulfides within the electrolyte potentially granted by the burgeoning 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 than 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 polysulfide shuttle mitigation methods such as additives or new 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 other researchers developing shuttle mitigation techniques.