Homogeneous Organometallic Catalysts for Improved Electrochemical Kinetics of Li–S Batteries

Abstract

The dissolution of polysulfides into electrolytes and sluggish electrochemical conversion kinetics primarily impede the practical realization of Li–S batteries. Homogeneous catalysis is an effective strategy to overcome the challenges involved under lean electrolyte conditions. Metallocenes, a class of organometallic compounds, hold promise to anchor and catalyze polysulfides. In this study, we used first-principles density functional theory (DFT) simulations to understand the role of metallocenes (using titanocene (TiCp2) as an archetypical example) as homogeneous catalysts in the electrolyte medium to suppress the shuttle effect and promote the reaction kinetics. The calculated electrochemical stability window of TiCp2 reveals that the composition is electrochemically inactive in the operating potential range of Li–S batteries and can thus be leveraged as an additive to expedite the reduction kinetics of soluble lithium polysulfides (LiPSs). We studied the detailed characteristic behavior of LiPS interactions with TiCp2 in both gas and solvent phases and the kinetics of elementary sulfur reduction reactions (SRRs). We found that TiCp2 provides adequate binding toward various LiPSs to mitigate the shuttle effect, and the structural integrity of LiPSs is well retained without any chemical decomposition. The catalyzing effect of TiCp2 is evident from the observed significant reduction in the SRR barriers, particularly for the rate-determining step, which is expected to favorably promote the deposition of Li2S on the cathode surface. We further propose a mechanistic scheme of TiCp2 homogeneous catalyst-boosted sulfur redox cycles. Overall, our simulations predicted moderate binding and improved kinetics of polysulfide chemistry with the metallocene-based homogeneous catalyst and are expected to lead to a paradigm shift in the design of organometallic additives for achieving high-performance metal–sulfur batteries.