The impact of polymeric binder on the morphology and performances of sulfur electrodes in lithium–sulfur batteries

An Expansion-Mitigant Binder for Stable Cycling of High-Loading Lithium-Sulfur Batteries

Lithium-sulfur batteries with high sulfur content and mass loading are promising energy storage technologies due to sulfur's exceptional theoretical energy density. However, in practice, their actual capacity drastically decays when the sulfur cathode is loaded to the commercially required levels of 4 mgsulfur cm-2 and above, significantly reducing the energy density. This reduction is due to the excessive formation of polysulfides during sulfur lithiation, which not only deteriorates battery performance through their detrimental shuttling but also results in substantial stress buildup due to their significantly larger volume compared to sulfur. To address these challenges, we have developed an approach to suppress lithium polysulfide shuttling by limiting the space for sulfur expansion while improving the Li+ ion diffusion. This was achieved through a straightforward but effective method to cross-link the organic binder used in sulfur electrodes. Specifically, PVDF, one of the most common binder materials for battery electrodes, was studied. The chemical, mechanical, and structural properties of the cross-linked PVDF binder were thoroughly investigated, compared with standard PVDF, and correlated to the achieved electrochemical performance of sulfur electrodes. As a result, sulfur cathodes with cross-linked PVDF binder exhibited prolonged cycle life compared to their standard counterparts. Moreover, using this expansion-mitigant binder, cathodes with areal sulfur loading of 4 mg cm-2 showed exceptional stability for more than 200 cycles and a Coulombic efficiency above 97%. This approach offers a promising avenue to alleviate the major roadblocks of lithium-sulfur battery commercialization while allowing the utilization of the commonly accessible and well-studied binder chemistries.

Polymers in Lithium-Sulfur Batteries

Lithium-sulfur batteries (LSBs) hold great promise as one of the next-generation power supplies for portable electronics and electric vehicles due to their ultrahigh energy density, cost effectiveness, and environmental benignity. However, their practical application has been impeded owing to the electronic insulation of sulfur and its intermediates, serious shuttle effect, large volume variation, and uncontrollable formation of lithium dendrites. Over the past decades, many pioneering strategies have been developed to address these issues via improving electrodes, electrolytes, separators and binders. Remarkably, polymers can be readily applied to all these aspects due to their structural designability, functional versatility, superior chemical stability and processability. Moreover, their lightweight and rich resource characteristics enable the production of LSBs with high-volume energy density at low cost. Surprisingly, there have been few reviews on development of polymers in LSBs. Herein, breakthroughs and future perspectives of emerging polymers in LSBs are scrutinized. Significant attention is centered on recent implementation of polymers in each component of LSBs with an emphasis on intrinsic mechanisms underlying their specific functions. The review offers a comprehensive overview of state-of-the-art polymers for LSBs, provides in-depth insights into addressing key challenges, and affords important resources for researchers working on electrochemical energy systems.

Dielectric Barrier Discharge (DBD) Plasma Coating of Sulfur for Mitigation of Capacity Fade in Lithium-Sulfur Batteries

Sulfur particles with a conductive polymer coating of poly(3,4-ethylene dioxythiophene) "PEDOT" were prepared by dielectric barrier discharge (DBD) plasma technology under atmospheric conditions (low temperature, ambient pressure). We report a solvent-free, low-cost, low-energy-consumption, safe, and low-risk process to make the material development and production compatible for sustainable technologies. Different coating protocols were developed to produce PEDOT-coated sulfur powders with electrical conductivity in the range of 10^-8-10^-5 S/cm. The raw sulfur powder (used as the reference) and (low-, optimum-, high-) PEDOT-coated sulfur powders were used to assemble lithium-sulfur (Li-S) cells with a high sulfur loading of ∼4.5 mg/cm². Long-term galvanostatic cycling at C/10 for 100 cycles showed that the capacity fade was mitigated by ∼30% for the cells containing the optimum-PEDOT-coated sulfur in comparison to the reference Li-S cells with raw sulfur. Rate capability, cyclic voltammetry, and electrochemical impedance analyzes confirmed the improved behavior of the PEDOT-coated sulfur as an active material for lithium-sulfur batteries. The Li-S cells containing optimum-PEDOT-coated sulfur showed the highest reproducibility of their electrochemical properties. A wide variety of bulk and surface characterization methods including conductivity analysis, X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and NMR spectroscopy were used to explain the chemical features and the superior behavior of Li-S cells using the optimum-PEDOT-coated sulfur material. Moreover, postmortem [SEM and Brunauer-Emmett-Teller (BET)] analyzes of uncoated and coated samples allowed us to exclude any significant effect at the electrode scale even after 70 cycles.