Redox Catalytic and Quasi-Solid Sulfur Conversion for High-Capacity Lean Lithium Sulfur Batteries

The practical deployment of lithium sulfur batteries demands stable cycling of high loading and dense sulfur cathodes under lean electrolyte conditions, which is very difficult to realize. We describe here a strategy of fabricating extremely dense sulfur cathodes, designed by integrating Mo₆S₈ nanoparticles as a multifunctional mediator with a Li-ion conducting binder and a high-performance Fe₃O₄@N-carbon sulfur host. The Mo₆S₈ nanoparticles have substantially faster Li-ion insertion kinetics compared with sulfur, and the produced LiMo₆S₈ particles have spontaneous redox reactivity with relevant polysulfide species (such as Li₄Mo₆S₈ + Li₂S₄ ↔ Li₃Mo₆S₈ + Li₂S, ΔG = -84 kJ mol^-1), which deliver a true redox catalytic sulfur conversion mechanism. In addition, LiMo₆S₈ particles strongly absorb polysulfide during battery cycling, which provides a quasi-solid sulfur conversion pathway and almost eliminated polysulfide dissolution. Such a pathway not only promotes growth of uniform Li₂S that can be readily charged back with nearly no overpotential, but also mitigates the polysulfide-induced Li metal corrosion issue. The combination of these benefits enables stable and high capacity cycling of dense sulfur cathodes under a low electrolyte to sulfur ratio (4.2 μL mg^-1), as demonstrated with cathodes with volumetric capacities of at least 1.3 Ah cm^-3 and capacity retentions of ∼80% for 300 cycles. Furthermore, stable cycling of batteries under a practically relevant N/P ratio of 2.4 is also demonstrated.